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            <h1>perlop</h1>


  <!--    -->
<ul><li><a href="#NAME">NAME
</a><li><a href="#DESCRIPTION">DESCRIPTION</a><ul><li><a href="#Operator-Precedence-and-Associativity">Operator Precedence and Associativity
  </a><li><a href="#Terms-and-List-Operators-(Leftward)">Terms and List Operators (Leftward)
  </a><li><a href="#The-Arrow-Operator">The Arrow Operator
  </a><li><a href="#Auto-increment-and-Auto-decrement">Auto-increment and Auto-decrement
     </a><li><a href="#Exponentiation">Exponentiation
  </a><li><a href="#Symbolic-Unary-Operators">Symbolic Unary Operators
 </a><li><a href="#Binding-Operators">Binding Operators
   </a><li><a href="#Multiplicative-Operators">Multiplicative Operators
</a><li><a href="#Additive-Operators">Additive Operators
</a><li><a href="#Shift-Operators">Shift Operators
  
   
   </a><li><a href="#Named-Unary-Operators">Named Unary Operators
</a><li><a href="#Relational-Operators">Relational Operators
 </a><li><a href="#Equality-Operators">Equality Operators
   </a><li><a href="#Smartmatch-Operator">Smartmatch Operator</a><li><a href="#Bitwise-And">Bitwise And
  </a><li><a href="#Bitwise-Or-and-Exclusive-Or">Bitwise Or and Exclusive Or
   
 </a><li><a href="#C-style-Logical-And">C-style Logical And
  </a><li><a href="#C-style-Logical-Or">C-style Logical Or
 </a><li><a href="#Logical-Defined-Or">Logical Defined-Or
 </a><li><a href="#Range-Operators">Range Operators
   </a><li><a href="#Conditional-Operator">Conditional Operator
   </a><li><a href="#Assignment-Operators">Assignment Operators
      
        
     </a><li><a href="#Comma-Operator">Comma Operator
  </a><li><a href="#List-Operators-(Rightward)">List Operators (Rightward)
 </a><li><a href="#Logical-Not">Logical Not
 </a><li><a href="#Logical-And">Logical And
 </a><li><a href="#Logical-or-and-Exclusive-Or">Logical or and Exclusive Or
 

 </a><li><a href="#C-Operators-Missing-From-Perl">C Operators Missing From Perl
  
 </a><li><a href="#Quote-and-Quote-like-Operators">Quote and Quote-like Operators
      
          
 </a><li><a href="#Regexp-Quote-Like-Operators">Regexp Quote-Like Operators
</a><li><a href="#Quote-Like-Operators">Quote-Like Operators
</a><li><a href="#Gory-details-of-parsing-quoted-constructs">Gory details of parsing quoted constructs
</a><li><a href="#I%2fO-Operators">I/O Operators
    
  </a><li><a href="#Constant-Folding">Constant Folding
 </a><li><a href="#No-ops">No-ops
 </a><li><a href="#Bitwise-String-Operators">Bitwise String Operators
    </a><li><a href="#Integer-Arithmetic">Integer Arithmetic
</a><li><a href="#Floating-point-Arithmetic">Floating-point Arithmetic</a><li><a href="#Bigger-Numbers">Bigger Numbers
</a></ul></ul><a name="NAME"></a><h1>NAME
</h1>
<p>perlop - Perl operators and precedence</p>
<a name="DESCRIPTION"></a><h1>DESCRIPTION</h1>
<p>In Perl, the operator determines what operation is performed,
independent of the type of the operands.  For example <code class="inline"><span class="i">$x</span> + <span class="i">$y</span></code>

is always a numeric addition, and if <code class="inline"><span class="i">$x</span></code>
 or <code class="inline"><span class="i">$y</span></code>
 do not contain
numbers, an attempt is made to convert them to numbers first.</p>
<p>This is in contrast to many other dynamic languages, where the
operation is determined by the type of the first argument.  It also
means that Perl has two versions of some operators, one for numeric
and one for string comparison.  For example <code class="inline"><span class="i">$x</span> == <span class="i">$y</span></code>
 compares
two numbers for equality, and <code class="inline"><span class="i">$x</span> <a class="l_k" href="functions/eq.html">eq</a> <span class="i">$y</span></code>
 compares two strings.</p>
<p>There are a few exceptions though: <code class="inline"><span class="w">x</span></code>
 can be either string
repetition or list repetition, depending on the type of the left
operand, and <code class="inline"><span class="i">&amp;</span></code>
, <code class="inline">|</code>, <code class="inline">^</code> and <code class="inline">~</code>
 can be either string or numeric bit
operations.</p>
<a name="Operator-Precedence-and-Associativity"></a><h2>Operator Precedence and Associativity
  </h2>
<p>Operator precedence and associativity work in Perl more or less like
they do in mathematics.</p>
<p><i>Operator precedence</i> means some operators are evaluated before
others.  For example, in <code class="inline"><span class="n">2</span> + <span class="n">4</span> * <span class="n">5</span></code>
, the multiplication has higher
precedence so <code class="inline"><span class="n">4</span> * <span class="n">5</span></code>
 is evaluated first yielding <code class="inline"><span class="n">2</span> + <span class="n">20</span> ==
<span class="n">22</span></code>
 and not <code class="inline"><span class="n">6</span> * <span class="n">5</span> == <span class="n">30</span></code>
.</p>
<p><i>Operator associativity</i> defines what happens if a sequence of the
same operators is used one after another: whether the evaluator will
evaluate the left operations first, or the right first.  For example, in
<code class="inline"><span class="n">8</span> - <span class="n">4</span> - <span class="n">2</span></code>
, subtraction is left associative so Perl evaluates the
expression left to right.  <code class="inline"><span class="n">8</span> - <span class="n">4</span></code>
 is evaluated first making the
expression <code class="inline"><span class="n">4</span> - <span class="n">2</span> == <span class="n">2</span></code>
 and not <code class="inline"><span class="n">8</span> - <span class="n">2</span> == <span class="n">6</span></code>
.</p>
<p>Perl operators have the following associativity and precedence,
listed from highest precedence to lowest.  Operators borrowed from
C keep the same precedence relationship with each other, even where
C's precedence is slightly screwy.  (This makes learning Perl easier
for C folks.)  With very few exceptions, these all operate on scalar
values only, not array values.</p>
<pre class="verbatim"><ol><li>    <span class="w">left</span>	<span class="w">terms</span> <a class="l_k" href="functions/and.html">and</a> <span class="w">list</span> <span class="w">operators</span> <span class="s">(</span><span class="w">leftward</span><span class="s">)</span></li><li>    <span class="w">left</span>	-&gt;</li><li>    <span class="w">nonassoc</span>	++ --</li><li>    <span class="w">right</span>	**</li><li>    <span class="w">right</span>	! ~ \ <a class="l_k" href="functions/and.html">and</a> <span class="w">unary</span> + <a class="l_k" href="functions/and.html">and</a> -</li><li>    <span class="w">left</span>	=~ !~</li><li>    <span class="w">left</span>	* <span class="q">/ % x</span></li><li>    <span class="q">    left	+ - .</span></li><li>    <span class="q">    left	&lt;&lt; &gt;&gt;</span></li><li>    <span class="q">    nonassoc	named unary operators</span></li><li>    <span class="q">    nonassoc	&lt; &gt; &lt;= &gt;= lt gt le ge</span></li><li>    <span class="q">    nonassoc	== != &lt;=&gt; eq ne cmp ~~</span></li><li>    <span class="q">    left	&amp;</span></li><li>    <span class="q">    left	| ^</span></li><li>    <span class="q">    left	&amp;&amp;</span></li><li>    <span class="q">    left	|| /</span>/</li><li>    <span class="w">nonassoc</span>	..  ...</li><li>    <span class="w">right</span>	?<span class="co">:</span></li><li>    <span class="w">right</span>	= += -= <span class="i">*=</span> <span class="w">etc</span>. <a class="l_k" href="functions/goto.html">goto</a> <a class="l_k" href="functions/last.html">last</a> <a class="l_k" href="functions/next.html">next</a> <a class="l_k" href="functions/redo.html">redo</a> <a class="l_k" href="functions/dump.html">dump</a></li><li>    <span class="w">left</span>	<span class="cm">,</span> <span class="cm">=&gt;</span></li><li>    <span class="w">nonassoc</span>	<span class="w">list</span> <span class="w">operators</span> <span class="s">(</span><span class="w">rightward</span><span class="s">)</span></li><li>    <span class="w">right</span>	<a class="l_k" href="functions/not.html">not</a></li><li>    <span class="w">left</span>	<a class="l_k" href="functions/and.html">and</a></li><li>    <span class="w">left</span>	<a class="l_k" href="functions/or.html">or</a> <a class="l_k" href="functions/xor.html">xor</a></li></ol></pre><p>In the following sections, these operators are covered in detail, in the
same order in which they appear in the table above.</p>
<p>Many operators can be overloaded for objects.  See <a href="overload.html">overload</a>.</p>
<a name="Terms-and-List-Operators-(Leftward)"></a><h2>Terms and List Operators (Leftward)
  </h2>
<p>A TERM has the highest precedence in Perl.  They include variables,
quote and quote-like operators, any expression in parentheses,
and any function whose arguments are parenthesized.  Actually, there
aren't really functions in this sense, just list operators and unary
operators behaving as functions because you put parentheses around
the arguments.  These are all documented in <a href="perlfunc.html">perlfunc</a>.</p>
<p>If any list operator (<code class="inline"><a class="l_k" href="functions/print.html">print()</a></code>, etc.) or any unary operator (<code class="inline"><a class="l_k" href="functions/chdir.html">chdir()</a></code>, etc.)
is followed by a left parenthesis as the next token, the operator and
arguments within parentheses are taken to be of highest precedence,
just like a normal function call.</p>
<p>In the absence of parentheses, the precedence of list operators such as
<code class="inline"><a class="l_k" href="functions/print.html">print</a></code>, <code class="inline"><a class="l_k" href="functions/sort.html">sort</a></code>, or <code class="inline"><a class="l_k" href="functions/chmod.html">chmod</a></code> is either very high or very low depending on
whether you are looking at the left side or the right side of the operator.
For example, in</p>
<pre class="verbatim"><ol><li>    <span class="i">@ary</span> = <span class="s">(</span><span class="n">1</span><span class="cm">,</span> <span class="n">3</span><span class="cm">,</span> <a class="l_k" href="functions/sort.html">sort</a> <span class="n">4</span><span class="cm">,</span> <span class="n">2</span><span class="s">)</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/print.html">print</a> <span class="i">@ary</span><span class="sc">;</span>		<span class="c"># prints 1324</span></li></ol></pre><p>the commas on the right of the <code class="inline"><a class="l_k" href="functions/sort.html">sort</a></code> are evaluated before the <code class="inline"><a class="l_k" href="functions/sort.html">sort</a></code>,
but the commas on the left are evaluated after.  In other words,
list operators tend to gobble up all arguments that follow, and
then act like a simple TERM with regard to the preceding expression.
Be careful with parentheses:</p>
<pre class="verbatim"><ol><li>    <span class="c"># These evaluate exit before doing the print:</span></li><li>    <a class="l_k" href="functions/print.html">print</a><span class="s">(</span><span class="i">$foo</span><span class="cm">,</span> <a class="l_k" href="functions/exit.html">exit</a><span class="s">)</span><span class="sc">;</span>	<span class="c"># Obviously not what you want.</span></li><li>    <a class="l_k" href="functions/print.html">print</a> <span class="i">$foo</span><span class="cm">,</span> <a class="l_k" href="functions/exit.html">exit</a><span class="sc">;</span>	<span class="c"># Nor is this.</span></li><li></li><li>    <span class="c"># These do the print before evaluating exit:</span></li><li>    <span class="s">(</span><a class="l_k" href="functions/print.html">print</a> <span class="i">$foo</span><span class="s">)</span><span class="cm">,</span> <a class="l_k" href="functions/exit.html">exit</a><span class="sc">;</span>	<span class="c"># This is what you want.</span></li><li>    <a class="l_k" href="functions/print.html">print</a><span class="s">(</span><span class="i">$foo</span><span class="s">)</span><span class="cm">,</span> <a class="l_k" href="functions/exit.html">exit</a><span class="sc">;</span>	<span class="c"># Or this.</span></li><li>    <a class="l_k" href="functions/print.html">print</a> <span class="s">(</span><span class="i">$foo</span><span class="s">)</span><span class="cm">,</span> <a class="l_k" href="functions/exit.html">exit</a><span class="sc">;</span>	<span class="c"># Or even this.</span></li></ol></pre><p>Also note that</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/print.html">print</a> <span class="s">(</span><span class="i">$foo</span> &amp; <span class="n">255</span><span class="s">)</span> + <span class="n">1</span><span class="cm">,</span> <span class="q">&quot;\n&quot;</span><span class="sc">;</span></li></ol></pre><p>probably doesn't do what you expect at first glance.  The parentheses
enclose the argument list for <code class="inline"><a class="l_k" href="functions/print.html">print</a></code> which is evaluated (printing
the result of <code class="inline"><span class="i">$foo</span> &amp; <span class="n">255</span></code>
).  Then one is added to the return value
of <code class="inline"><a class="l_k" href="functions/print.html">print</a></code> (usually 1).  The result is something like this:</p>
<pre class="verbatim"><ol><li>    <span class="n">1</span> + <span class="n">1</span><span class="cm">,</span> <span class="q">&quot;\n&quot;</span><span class="sc">;</span>    <span class="c"># Obviously not what you meant.</span></li></ol></pre><p>To do what you meant properly, you must write:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/print.html">print</a><span class="s">(</span><span class="s">(</span><span class="i">$foo</span> &amp; <span class="n">255</span><span class="s">)</span> + <span class="n">1</span><span class="cm">,</span> <span class="q">&quot;\n&quot;</span><span class="s">)</span><span class="sc">;</span></li></ol></pre><p>See <a href="#Named-Unary-Operators">Named Unary Operators</a> for more discussion of this.</p>
<p>Also parsed as terms are the <code class="inline"><a class="l_k" href="functions/do.html">do</a> <span class="s">{</span><span class="s">}</span></code>
 and <code class="inline"><a class="l_k" href="functions/eval.html">eval</a> <span class="s">{</span><span class="s">}</span></code>
 constructs, as
well as subroutine and method calls, and the anonymous
constructors <code class="inline"><span class="s">[</span><span class="s">]</span></code>
 and <code class="inline"><span class="s">{</span><span class="s">}</span></code>
.</p>
<p>See also <a href="#Quote-and-Quote-like-Operators">Quote and Quote-like Operators</a> toward the end of this section,
as well as <a href="#I%2fO-Operators">I/O Operators</a>.</p>
<a name="The-Arrow-Operator"></a><h2>The Arrow Operator
  </h2>
<p>"<code class="inline">-&gt;</code>
" is an infix dereference operator, just as it is in C
and C++.  If the right side is either a <code class="inline"><span class="s">[</span>...<span class="s">]</span></code>
, <code class="inline"><span class="s">{</span>...<span class="s">}</span></code>
, or a
<code class="inline"><span class="s">(</span>...<span class="s">)</span></code>
 subscript, then the left side must be either a hard or
symbolic reference to an array, a hash, or a subroutine respectively.
(Or technically speaking, a location capable of holding a hard
reference, if it's an array or hash reference being used for
assignment.)  See <a href="perlreftut.html">perlreftut</a> and <a href="perlref.html">perlref</a>.</p>
<p>Otherwise, the right side is a method name or a simple scalar
variable containing either the method name or a subroutine reference,
and the left side must be either an object (a blessed reference)
or a class name (that is, a package name).  See <a href="perlobj.html">perlobj</a>.</p>
<p>The dereferencing cases (as opposed to method-calling cases) are
somewhat extended by the <code class="inline"><span class="w">postderef</span></code>
 feature.  For the
details of that feature, consult <a href="perlref.html#Postfix-Dereference-Syntax">Postfix Dereference Syntax in perlref</a>.</p>
<a name="Auto-increment-and-Auto-decrement"></a><h2>Auto-increment and Auto-decrement
     </h2>
<p><code class="inline"><span class="q">&quot;++&quot;</span></code>
 and <code class="inline"><span class="q">&quot;--&quot;</span></code>
 work as in C.  That is, if placed before a variable,
they increment or decrement the variable by one before returning the
value, and if placed after, increment or decrement after returning the
value.</p>
<pre class="verbatim"><ol><li>    <span class="i">$i</span> = <span class="n">0</span><span class="sc">;</span>  <span class="i">$j</span> = <span class="n">0</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/print.html">print</a> <span class="i">$i</span>++<span class="sc">;</span>  <span class="c"># prints 0</span></li><li>    <a class="l_k" href="functions/print.html">print</a> ++<span class="i">$j</span><span class="sc">;</span>  <span class="c"># prints 1</span></li></ol></pre><p>Note that just as in C, Perl doesn't define <b>when</b> the variable is
incremented or decremented.  You just know it will be done sometime
before or after the value is returned.  This also means that modifying
a variable twice in the same statement will lead to undefined behavior.
Avoid statements like:</p>
<pre class="verbatim"><ol><li>    <span class="i">$i</span> = <span class="i">$i</span> ++<span class="sc">;</span></li><li>    <a class="l_k" href="functions/print.html">print</a> ++ <span class="i">$i</span> + <span class="i">$i</span> ++<span class="sc">;</span></li></ol></pre><p>Perl will not guarantee what the result of the above statements is.</p>
<p>The auto-increment operator has a little extra builtin magic to it.  If
you increment a variable that is numeric, or that has ever been used in
a numeric context, you get a normal increment.  If, however, the
variable has been used in only string contexts since it was set, and
has a value that is not the empty string and matches the pattern
<code class="inline"><span class="q">/^[a-zA-Z]*[0-9]*\z/</span></code>
, the increment is done as a string, preserving each
character within its range, with carry:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/print.html">print</a> ++<span class="s">(</span><span class="i">$foo</span> = <span class="q">&quot;99&quot;</span><span class="s">)</span><span class="sc">;</span>	<span class="c"># prints &quot;100&quot;</span></li><li>    <a class="l_k" href="functions/print.html">print</a> ++<span class="s">(</span><span class="i">$foo</span> = <span class="q">&quot;a0&quot;</span><span class="s">)</span><span class="sc">;</span>	<span class="c"># prints &quot;a1&quot;</span></li><li>    <a class="l_k" href="functions/print.html">print</a> ++<span class="s">(</span><span class="i">$foo</span> = <span class="q">&quot;Az&quot;</span><span class="s">)</span><span class="sc">;</span>	<span class="c"># prints &quot;Ba&quot;</span></li><li>    <a class="l_k" href="functions/print.html">print</a> ++<span class="s">(</span><span class="i">$foo</span> = <span class="q">&quot;zz&quot;</span><span class="s">)</span><span class="sc">;</span>	<span class="c"># prints &quot;aaa&quot;</span></li></ol></pre><p><code class="inline"><a class="l_k" href="functions/undef.html">undef</a></code> is always treated as numeric, and in particular is changed
to <code class="inline"><span class="n">0</span></code>
 before incrementing (so that a post-increment of an undef value
will return <code class="inline"><span class="n">0</span></code>
 rather than <code class="inline"><a class="l_k" href="functions/undef.html">undef</a></code>).</p>
<p>The auto-decrement operator is not magical.</p>
<a name="Exponentiation"></a><h2>Exponentiation
  </h2>
<p>Binary <code class="inline"><span class="q">&quot;**&quot;</span></code>
 is the exponentiation operator.  It binds even more
tightly than unary minus, so <code class="inline"><span class="n">-2</span>**<span class="n">4</span></code>
 is <code class="inline">-<span class="s">(</span><span class="n">2</span>**<span class="n">4</span><span class="s">)</span></code>
, not <code class="inline"><span class="s">(</span><span class="n">-2</span><span class="s">)</span>**<span class="n">4</span></code>
.
(This is
implemented using C's <code class="inline"><span class="i">pow</span><span class="s">(</span><span class="n">3</span><span class="s">)</span></code>
 function, which actually works on doubles
internally.)</p>
<p>Note that certain exponentiation expressions are ill-defined:
these include <code class="inline"><span class="n">0</span>**<span class="n">0</span></code>
, <code class="inline"><span class="n">1</span>**<span class="w">Inf</span></code>
, and <code class="inline"><span class="w">Inf</span>**<span class="n">0</span></code>
.  Do not expect
any particular results from these special cases, the results
are platform-dependent.</p>
<a name="Symbolic-Unary-Operators"></a><h2>Symbolic Unary Operators
 </h2>
<p>Unary <code class="inline"><span class="q">&quot;!&quot;</span></code>
 performs logical negation, that is, "not".  See also
<a href="#Logical-Not">not </a> for a lower precedence version of this.
</p>
<p>Unary <code class="inline"><span class="q">&quot;-&quot;</span></code>
 performs arithmetic negation if the operand is numeric,
including any string that looks like a number.  If the operand is
an identifier, a string consisting of a minus sign concatenated
with the identifier is returned.  Otherwise, if the string starts
with a plus or minus, a string starting with the opposite sign is
returned.  One effect of these rules is that <code class="inline">-<span class="w">bareword</span></code>
 is equivalent
to the string <code class="inline"><span class="q">&quot;-bareword&quot;</span></code>
.  If, however, the string begins with a
non-alphabetic character (excluding <code class="inline"><span class="q">&quot;+&quot;</span></code>
 or <code class="inline"><span class="q">&quot;-&quot;</span></code>
), Perl will attempt
to convert
the string to a numeric, and the arithmetic negation is performed.  If the
string cannot be cleanly converted to a numeric, Perl will give the warning
<b>Argument "the string" isn't numeric in negation (-) at ...</b>.
 </p>
<p>Unary <code class="inline"><span class="q">&quot;~&quot;</span></code>
 performs bitwise negation, that is, 1's complement.  For
example, <code class="inline"><span class="n">0666</span> &amp; ~<span class="n">027</span></code>
 is 0640.  (See also <a href="#Integer-Arithmetic">Integer Arithmetic</a> and
<a href="#Bitwise-String-Operators">Bitwise String Operators</a>.)  Note that the width of the result is
platform-dependent: <code class="inline">~<span class="n">0</span></code>
 is 32 bits wide on a 32-bit platform, but 64
bits wide on a 64-bit platform, so if you are expecting a certain bit
width, remember to use the <code class="inline"><span class="q">&quot;&amp;&quot;</span></code>
 operator to mask off the excess bits.
 </p>
<p>When complementing strings, if all characters have ordinal values under
256, then their complements will, also.  But if they do not, all
characters will be in either 32- or 64-bit complements, depending on your
architecture.  So for example, <code class="inline">~<span class="q">&quot;\x{3B1}&quot;</span></code>
 is <code class="inline"><span class="q">&quot;\x{FFFF_FC4E}&quot;</span></code>
 on
32-bit machines and <code class="inline"><span class="q">&quot;\x{FFFF_FFFF_FFFF_FC4E}&quot;</span></code>
 on 64-bit machines.</p>
<p>If the experimental "bitwise" feature is enabled via <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">feature</span>
<span class="q">&#39;bitwise&#39;</span></code>
, then unary <code class="inline"><span class="q">&quot;~&quot;</span></code>
 always treats its argument as a number, and an
alternate form of the operator, <code class="inline"><span class="q">&quot;~.&quot;</span></code>
, always treats its argument as a
string.  So <code class="inline">~<span class="n">0</span></code>
 and <code class="inline">~<span class="q">&quot;0&quot;</span></code>
 will both give 2**32-1 on 32-bit platforms,
whereas <code class="inline">~<span class="n">.0</span></code>
 and <code class="inline">~.<span class="q">&quot;0&quot;</span></code>
 will both yield <code class="inline"><span class="q">&quot;\xff&quot;</span></code>
.  This feature
produces a warning unless you use <code class="inline"><a class="l_k" href="functions/no.html">no</a> <span class="w">warnings</span> <span class="q">&#39;experimental::bitwise&#39;</span></code>
.</p>
<p>Unary <code class="inline"><span class="q">&quot;+&quot;</span></code>
 has no effect whatsoever, even on strings.  It is useful
syntactically for separating a function name from a parenthesized expression
that would otherwise be interpreted as the complete list of function
arguments.  (See examples above under <a href="#Terms-and-List-Operators-(Leftward)">Terms and List Operators (Leftward)</a>.)
</p>
<p>Unary <code class="inline"><span class="q">&quot;\&quot;</span></code>
 creates a reference to whatever follows it.  See <a href="perlreftut.html">perlreftut</a>
and <a href="perlref.html">perlref</a>.  Do not confuse this behavior with the behavior of
backslash within a string, although both forms do convey the notion
of protecting the next thing from interpolation.
  </p>
<a name="Binding-Operators"></a><h2>Binding Operators
   </h2>
<p>Binary <code class="inline"><span class="q">&quot;=~&quot;</span></code>
 binds a scalar expression to a pattern match.  Certain operations
search or modify the string <code class="inline"><span class="i">$_</span></code>
 by default.  This operator makes that kind
of operation work on some other string.  The right argument is a search
pattern, substitution, or transliteration.  The left argument is what is
supposed to be searched, substituted, or transliterated instead of the default
<code class="inline"><span class="i">$_</span></code>
.  When used in scalar context, the return value generally indicates the
success of the operation.  The exceptions are substitution (<code class="inline"><a class="l_k" href="functions/s.html">s///</a></code>)
and transliteration (<code class="inline"><a class="l_k" href="functions/y.html">y///</a></code>) with the <code class="inline"><span class="q">/r</span></code>
 (non-destructive) option,
which cause the <b>r</b>eturn value to be the result of the substitution.
Behavior in list context depends on the particular operator.
See <a href="#Regexp-Quote-Like-Operators">Regexp Quote-Like Operators</a> for details and <a href="perlretut.html">perlretut</a> for
examples using these operators.</p>
<p>If the right argument is an expression rather than a search pattern,
substitution, or transliteration, it is interpreted as a search pattern at run
time.  Note that this means that its
contents will be interpolated twice, so</p>
<pre class="verbatim"><ol><li>    <span class="q">&#39;\\&#39;</span> =~ <span class="q">q&#39;\\&#39;</span><span class="sc">;</span></li></ol></pre><p>is not ok, as the regex engine will end up trying to compile the
pattern <code class="inline">\</code>
, which it will consider a syntax error.</p>
<p>Binary <code class="inline"><span class="q">&quot;!~&quot;</span></code>
 is just like <code class="inline"><span class="q">&quot;=~&quot;</span></code>
 except the return value is negated in
the logical sense.</p>
<p>Binary <code class="inline"><span class="q">&quot;!~&quot;</span></code>
 with a non-destructive substitution (<code class="inline"><a class="l_k" href="functions/s.html">s///r</a></code>) or transliteration
(<code class="inline"><a class="l_k" href="functions/y.html">y///r</a></code>) is a syntax error.</p>
<a name="Multiplicative-Operators"></a><h2>Multiplicative Operators
</h2>
<p>Binary <code class="inline"><span class="q">&quot;*&quot;</span></code>
 multiplies two numbers.
</p>
<p>Binary <code class="inline"><span class="q">&quot;/&quot;</span></code>
 divides two numbers.
 </p>
<p>Binary <code class="inline"><span class="q">&quot;%&quot;</span></code>
 is the modulo operator, which computes the division
remainder of its first argument with respect to its second argument.
Given integer
operands <code class="inline"><span class="i">$m</span></code>
 and <code class="inline"><span class="i">$n</span></code>
: If <code class="inline"><span class="i">$n</span></code>
 is positive, then <code class="inline"><span class="i">$m</span> % <span class="i">$n</span></code>
 is
<code class="inline"><span class="i">$m</span></code>
 minus the largest multiple of <code class="inline"><span class="i">$n</span></code>
 less than or equal to
<code class="inline"><span class="i">$m</span></code>
.  If <code class="inline"><span class="i">$n</span></code>
 is negative, then <code class="inline"><span class="i">$m</span> % <span class="i">$n</span></code>
 is <code class="inline"><span class="i">$m</span></code>
 minus the
smallest multiple of <code class="inline"><span class="i">$n</span></code>
 that is not less than <code class="inline"><span class="i">$m</span></code>
 (that is, the
result will be less than or equal to zero).  If the operands
<code class="inline"><span class="i">$m</span></code>
 and <code class="inline"><span class="i">$n</span></code>
 are floating point values and the absolute value of
<code class="inline"><span class="i">$n</span></code>
 (that is <code class="inline"><a class="l_k" href="functions/abs.html">abs($n)</a></code>) is less than <code class="inline"><span class="s">(</span><span class="w">UV_MAX</span> + <span class="n">1</span><span class="s">)</span></code>
, only
the integer portion of <code class="inline"><span class="i">$m</span></code>
 and <code class="inline"><span class="i">$n</span></code>
 will be used in the operation
(Note: here <code class="inline"><span class="w">UV_MAX</span></code>
 means the maximum of the unsigned integer type).
If the absolute value of the right operand (<code class="inline"><a class="l_k" href="functions/abs.html">abs($n)</a></code>) is greater than
or equal to <code class="inline"><span class="s">(</span><span class="w">UV_MAX</span> + <span class="n">1</span><span class="s">)</span></code>
, <code class="inline"><span class="q">&quot;%&quot;</span></code>
 computes the floating-point remainder
<code class="inline"><span class="i">$r</span></code>
 in the equation <code class="inline"><span class="s">(</span><span class="i">$r</span> = <span class="i">$m</span> - <span class="i">$i</span>*<span class="i">$n</span><span class="s">)</span></code>
 where <code class="inline"><span class="i">$i</span></code>
 is a certain
integer that makes <code class="inline"><span class="i">$r</span></code>
 have the same sign as the right operand
<code class="inline"><span class="i">$n</span></code>
 (<b>not</b> as the left operand <code class="inline"><span class="i">$m</span></code>
 like C function <code class="inline"><span class="i">fmod</span><span class="s">(</span><span class="s">)</span></code>
)
and the absolute value less than that of <code class="inline"><span class="i">$n</span></code>
.
Note that when <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">integer</span></code>
 is in scope, <code class="inline"><span class="q">&quot;%&quot;</span></code>
 gives you direct access
to the modulo operator as implemented by your C compiler.  This
operator is not as well defined for negative operands, but it will
execute faster.
   </p>
<p>Binary <code class="inline"><span class="q">&quot;x&quot;</span></code>
 is the repetition operator.  In scalar context or if the left
operand is not enclosed in parentheses, it returns a string consisting
of the left operand repeated the number of times specified by the right
operand.  In list context, if the left operand is enclosed in
parentheses or is a list formed by <code class="inline">qw/<i>STRING</i>/</code>, it repeats the list.
If the right operand is zero or negative (raising a warning on
negative), it returns an empty string
or an empty list, depending on the context.
</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/print.html">print</a> <span class="q">&#39;-&#39;</span> x <span class="n">80</span><span class="sc">;</span>		<span class="c"># print row of dashes</span></li><li></li><li>    <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;\t&quot;</span> x <span class="s">(</span><span class="i">$tab</span>/<span class="n">8</span><span class="s">)</span><span class="cm">,</span> <span class="q">&#39; &#39;</span> x <span class="s">(</span><span class="i">$tab</span>%<span class="n">8</span><span class="s">)</span><span class="sc">;</span>	<span class="c"># tab over</span></li><li></li><li>    <span class="i">@ones</span> = <span class="s">(</span><span class="n">1</span><span class="s">)</span> x <span class="n">80</span><span class="sc">;</span>		<span class="c"># a list of 80 1&#39;s</span></li><li>    <span class="i">@ones</span> = <span class="s">(</span><span class="n">5</span><span class="s">)</span> x <span class="i">@ones</span><span class="sc">;</span>	<span class="c"># set all elements to 5</span></li></ol></pre><a name="Additive-Operators"></a><h2>Additive Operators
</h2>
<p>Binary <code class="inline"><span class="q">&quot;+&quot;</span></code>
 returns the sum of two numbers.
</p>
<p>Binary <code class="inline"><span class="q">&quot;-&quot;</span></code>
 returns the difference of two numbers.
</p>
<p>Binary <code class="inline"><span class="q">&quot;.&quot;</span></code>
 concatenates two strings.
 
   </p>
<a name="Shift-Operators"></a><h2>Shift Operators
  
   
   </h2>
<p>Binary <code class="inline"><span class="q">&quot;&lt;&lt;&quot;</span></code>
 returns the value of its left argument shifted left by the
number of bits specified by the right argument.  Arguments should be
integers.  (See also <a href="#Integer-Arithmetic">Integer Arithmetic</a>.)</p>
<p>Binary <code class="inline"><span class="q">&quot;&gt;&gt;&quot;</span></code>
 returns the value of its left argument shifted right by
the number of bits specified by the right argument.  Arguments should
be integers.  (See also <a href="#Integer-Arithmetic">Integer Arithmetic</a>.)</p>
<p>If <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">integer</span></code>
 (see <a href="#Integer-Arithmetic">Integer Arithmetic</a>) is in force then
signed C integers are used (<i>arithmetic shift</i>), otherwise unsigned C
integers are used (<i>logical shift</i>), even for negative shiftees.
In arithmetic right shift the sign bit is replicated on the left,
in logical shift zero bits come in from the left.</p>
<p>Either way, the implementation isn't going to generate results larger
than the size of the integer type Perl was built with (32 bits or 64 bits).</p>
<p>Shifting by negative number of bits means the reverse shift: left
shift becomes right shift, right shift becomes left shift.  This is
unlike in C, where negative shift is undefined.</p>
<p>Shifting by more bits than the size of the integers means most of the
time zero (all bits fall off), except that under <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">integer</span></code>

right overshifting a negative shiftee results in -1.  This is unlike
in C, where shifting by too many bits is undefined.  A common C
behavior is "shift by modulo wordbits", so that for example</p>
<pre class="verbatim"><ol><li>    <span class="n">1</span> &gt;&gt; <span class="n">64</span> == <span class="n">1</span> &gt;&gt; <span class="s">(</span><span class="n">64</span> % <span class="n">64</span><span class="s">)</span> == <span class="n">1</span> &gt;&gt; <span class="n">0</span> == <span class="n">1</span>  <span class="c"># Common C behavior.</span></li></ol></pre><p>but that is completely accidental.</p>
<p>If you get tired of being subject to your platform's native integers,
the <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">bigint</span></code>
 pragma neatly sidesteps the issue altogether:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/print.html">print</a> <span class="n">20</span> &lt;&lt; <span class="n">20</span><span class="sc">;</span>  <span class="c"># 20971520</span></li><li>    <a class="l_k" href="functions/print.html">print</a> <span class="n">20</span> &lt;&lt; <span class="n">40</span><span class="sc">;</span>  <span class="c"># 5120 on 32-bit machines,</span></li><li>                     <span class="c"># 21990232555520 on 64-bit machines</span></li><li>    <a class="l_k" href="functions/use.html">use</a> <span class="w">bigint</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/print.html">print</a> <span class="n">20</span> &lt;&lt; <span class="n">100</span><span class="sc">;</span> <span class="c"># 25353012004564588029934064107520</span></li></ol></pre><a name="Named-Unary-Operators"></a><h2>Named Unary Operators
</h2>
<p>The various named unary operators are treated as functions with one
argument, with optional parentheses.</p>
<p>If any list operator (<code class="inline"><a class="l_k" href="functions/print.html">print()</a></code>, etc.) or any unary operator (<code class="inline"><a class="l_k" href="functions/chdir.html">chdir()</a></code>, etc.)
is followed by a left parenthesis as the next token, the operator and
arguments within parentheses are taken to be of highest precedence,
just like a normal function call.  For example,
because named unary operators are higher precedence than <code class="inline">||</code>:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/chdir.html">chdir</a> <span class="i">$foo</span>    || <a class="l_k" href="functions/die.html">die</a><span class="sc">;</span>	<span class="c"># (chdir $foo) || die</span></li><li>    <a class="l_k" href="functions/chdir.html">chdir</a><span class="s">(</span><span class="i">$foo</span><span class="s">)</span>   || <a class="l_k" href="functions/die.html">die</a><span class="sc">;</span>	<span class="c"># (chdir $foo) || die</span></li><li>    <a class="l_k" href="functions/chdir.html">chdir</a> <span class="s">(</span><span class="i">$foo</span><span class="s">)</span>  || <a class="l_k" href="functions/die.html">die</a><span class="sc">;</span>	<span class="c"># (chdir $foo) || die</span></li><li>    <a class="l_k" href="functions/chdir.html">chdir</a> +<span class="s">(</span><span class="i">$foo</span><span class="s">)</span> || <a class="l_k" href="functions/die.html">die</a><span class="sc">;</span>	<span class="c"># (chdir $foo) || die</span></li></ol></pre><p>but, because <code class="inline"><span class="q">&quot;*&quot;</span></code>
 is higher precedence than named operators:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/chdir.html">chdir</a> <span class="i">$foo</span> * <span class="n">20</span><span class="sc">;</span>	<span class="c"># chdir ($foo * 20)</span></li><li>    <a class="l_k" href="functions/chdir.html">chdir</a><span class="s">(</span><span class="i">$foo</span><span class="s">)</span> * <span class="n">20</span><span class="sc">;</span>	<span class="c"># (chdir $foo) * 20</span></li><li>    <a class="l_k" href="functions/chdir.html">chdir</a> <span class="s">(</span><span class="i">$foo</span><span class="s">)</span> * <span class="n">20</span><span class="sc">;</span>	<span class="c"># (chdir $foo) * 20</span></li><li>    <a class="l_k" href="functions/chdir.html">chdir</a> +<span class="s">(</span><span class="i">$foo</span><span class="s">)</span> * <span class="n">20</span><span class="sc">;</span>	<span class="c"># chdir ($foo * 20)</span></li><li></li><li>    <a class="l_k" href="functions/rand.html">rand</a> <span class="n">10</span> * <span class="n">20</span><span class="sc">;</span>	<span class="c"># rand (10 * 20)</span></li><li>    <a class="l_k" href="functions/rand.html">rand</a><span class="s">(</span><span class="n">10</span><span class="s">)</span> * <span class="n">20</span><span class="sc">;</span>	<span class="c"># (rand 10) * 20</span></li><li>    <a class="l_k" href="functions/rand.html">rand</a> <span class="s">(</span><span class="n">10</span><span class="s">)</span> * <span class="n">20</span><span class="sc">;</span>	<span class="c"># (rand 10) * 20</span></li><li>    <a class="l_k" href="functions/rand.html">rand</a> +<span class="s">(</span><span class="n">10</span><span class="s">)</span> * <span class="n">20</span><span class="sc">;</span>	<span class="c"># rand (10 * 20)</span></li></ol></pre><p>Regarding precedence, the filetest operators, like <code class="inline">-f</code>
, <code class="inline">-M</code>
, etc. are
treated like named unary operators, but they don't follow this functional
parenthesis rule.  That means, for example, that <code class="inline">-f<span class="s">(</span><span class="i">$file</span><span class="s">)</span>.<span class="q">&quot;.bak&quot;</span></code>
 is
equivalent to <code class="inline">-f <span class="q">&quot;$file.bak&quot;</span></code>
.
  </p>
<p>See also <a href="#Terms-and-List-Operators-(Leftward)">Terms and List Operators (Leftward)</a>.</p>
<a name="Relational-Operators"></a><h2>Relational Operators
 </h2>
<p>Perl operators that return true or false generally return values
that can be safely used as numbers.  For example, the relational
operators in this section and the equality operators in the next
one return <code class="inline"><span class="n">1</span></code>
 for true and a special version of the defined empty
string, <code class="inline"><span class="q">&quot;&quot;</span></code>
, which counts as a zero but is exempt from warnings
about improper numeric conversions, just as <code class="inline"><span class="q">&quot;0 but true&quot;</span></code>
 is.</p>
<p>Binary <code class="inline"><span class="q">&quot;&lt;&quot;</span></code>
 returns true if the left argument is numerically less than
the right argument.
</p>
<p>Binary <code class="inline"><span class="q">&quot;&gt;&quot;</span></code>
 returns true if the left argument is numerically greater
than the right argument.
</p>
<p>Binary <code class="inline"><span class="q">&quot;&lt;=&quot;</span></code>
 returns true if the left argument is numerically less than
or equal to the right argument.
</p>
<p>Binary <code class="inline"><span class="q">&quot;&gt;=&quot;</span></code>
 returns true if the left argument is numerically greater
than or equal to the right argument.
</p>
<p>Binary <code class="inline"><span class="q">&quot;lt&quot;</span></code>
 returns true if the left argument is stringwise less than
the right argument.
</p>
<p>Binary <code class="inline"><span class="q">&quot;gt&quot;</span></code>
 returns true if the left argument is stringwise greater
than the right argument.
</p>
<p>Binary <code class="inline"><span class="q">&quot;le&quot;</span></code>
 returns true if the left argument is stringwise less than
or equal to the right argument.
</p>
<p>Binary <code class="inline"><span class="q">&quot;ge&quot;</span></code>
 returns true if the left argument is stringwise greater
than or equal to the right argument.
</p>
<a name="Equality-Operators"></a><h2>Equality Operators
   </h2>
<p>Binary <code class="inline"><span class="q">&quot;==&quot;</span></code>
 returns true if the left argument is numerically equal to
the right argument.
</p>
<p>Binary <code class="inline"><span class="q">&quot;!=&quot;</span></code>
 returns true if the left argument is numerically not equal
to the right argument.
</p>
<p>Binary <code class="inline"><span class="q">&quot;&lt;=&gt;&quot;</span></code>
 returns -1, 0, or 1 depending on whether the left
argument is numerically less than, equal to, or greater than the right
argument.  If your platform supports <code class="inline"><span class="w">NaN</span></code>
's (not-a-numbers) as numeric
values, using them with <code class="inline"><span class="q">&quot;&lt;=&gt;&quot;</span></code>
 returns undef.  <code class="inline"><span class="w">NaN</span></code>
 is not
<code class="inline"><span class="q">&quot;&lt;&quot;</span></code>
, <code class="inline"><span class="q">&quot;==&quot;</span></code>
, <code class="inline"><span class="q">&quot;&gt;&quot;</span></code>
, <code class="inline"><span class="q">&quot;&lt;=&quot;</span></code>
 or <code class="inline"><span class="q">&quot;&gt;=&quot;</span></code>
 anything
(even <code class="inline"><span class="w">NaN</span></code>
), so those 5 return false.  <code class="inline"><span class="w">NaN</span> != <span class="w">NaN</span></code>
 returns
true, as does <code class="inline"><span class="w">NaN</span> !=</code>
 <i>anything else</i>.  If your platform doesn't
support <code class="inline"><span class="w">NaN</span></code>
's then <code class="inline"><span class="w">NaN</span></code>
 is just a string with numeric value 0.

</p>
<pre class="verbatim"><ol><li>    <span class="i">$ perl</span> -<a class="l_k" href="functions/le.html">le</a> <span class="q">&#39;$x = &quot;NaN&quot;; print &quot;No NaN support here&quot; if $x == $x&#39;</span></li><li>    <span class="i">$ perl</span> -<a class="l_k" href="functions/le.html">le</a> <span class="q">&#39;$x = &quot;NaN&quot;; print &quot;NaN support here&quot; if $x != $x&#39;</span></li></ol></pre><p>(Note that the <a href="bigint.html">bigint</a>, <a href="bigrat.html">bigrat</a>, and <a href="bignum.html">bignum</a> pragmas all
support <code class="inline"><span class="q">&quot;NaN&quot;</span></code>
.)</p>
<p>Binary <code class="inline"><span class="q">&quot;eq&quot;</span></code>
 returns true if the left argument is stringwise equal to
the right argument.
</p>
<p>Binary <code class="inline"><span class="q">&quot;ne&quot;</span></code>
 returns true if the left argument is stringwise not equal
to the right argument.
</p>
<p>Binary <code class="inline"><span class="q">&quot;cmp&quot;</span></code>
 returns -1, 0, or 1 depending on whether the left
argument is stringwise less than, equal to, or greater than the right
argument.
</p>
<p>Binary <code class="inline"><span class="q">&quot;~~&quot;</span></code>
 does a smartmatch between its arguments.  Smart matching
is described in the next section.
</p>
<p><code class="inline"><span class="q">&quot;lt&quot;</span></code>
, <code class="inline"><span class="q">&quot;le&quot;</span></code>
, <code class="inline"><span class="q">&quot;ge&quot;</span></code>
, <code class="inline"><span class="q">&quot;gt&quot;</span></code>
 and <code class="inline"><span class="q">&quot;cmp&quot;</span></code>
 use the collation (sort)
order specified by the current <code class="inline"><span class="w">LC_COLLATE</span></code>
 locale if a <code class="inline"><a class="l_k" href="functions/use.html">use</a>
<span class="w">locale</span></code>
 form that includes collation is in effect.  See <a href="perllocale.html">perllocale</a>.
Do not mix these with Unicode,
only use them with legacy 8-bit locale encodings.
The standard <code class="inline"><a href="Unicode/Collate.html">Unicode::Collate</a></code> and
<code class="inline"><a href="Unicode/Collate/Locale.html">Unicode::Collate::Locale</a></code> modules offer much more powerful
solutions to collation issues.</p>
<p>For case-insensitive comparisions, look at the <a href="functions/fc.html">fc</a> case-folding
function, available in Perl v5.16 or later:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/if.html">if</a> <span class="s">(</span> <span class="i">fc</span><span class="s">(</span><span class="i">$x</span><span class="s">)</span> <a class="l_k" href="functions/eq.html">eq</a> <span class="i">fc</span><span class="s">(</span><span class="i">$y</span><span class="s">)</span> <span class="s">)</span> <span class="s">{</span> ... <span class="s">}</span></li></ol></pre><a name="Smartmatch-Operator"></a><h2>Smartmatch Operator</h2>
<p>First available in Perl 5.10.1 (the 5.10.0 version behaved differently),
binary <code class="inline">~~</code>
 does a "smartmatch" between its arguments.  This is mostly
used implicitly in the <code class="inline">when</code>
 construct described in <a href="perlsyn.html">perlsyn</a>, although
not all <code class="inline">when</code>
 clauses call the smartmatch operator.  Unique among all of
Perl's operators, the smartmatch operator can recurse.  The smartmatch
operator is <a href="perlpolicy.html#experimental">experimental</a> and its behavior is
subject to change.</p>
<p>It is also unique in that all other Perl operators impose a context
(usually string or numeric context) on their operands, autoconverting
those operands to those imposed contexts.  In contrast, smartmatch
<i>infers</i> contexts from the actual types of its operands and uses that
type information to select a suitable comparison mechanism.</p>
<p>The <code class="inline">~~</code>
 operator compares its operands "polymorphically", determining how
to compare them according to their actual types (numeric, string, array,
hash, etc.)  Like the equality operators with which it shares the same
precedence, <code class="inline">~~</code>
 returns 1 for true and <code class="inline"><span class="q">&quot;&quot;</span></code>
 for false.  It is often best
read aloud as "in", "inside of", or "is contained in", because the left
operand is often looked for <i>inside</i> the right operand.  That makes the
order of the operands to the smartmatch operand often opposite that of
the regular match operator.  In other words, the "smaller" thing is usually
placed in the left operand and the larger one in the right.</p>
<p>The behavior of a smartmatch depends on what type of things its arguments
are, as determined by the following table.  The first row of the table
whose types apply determines the smartmatch behavior.  Because what
actually happens is mostly determined by the type of the second operand,
the table is sorted on the right operand instead of on the left.</p>
<pre class="verbatim"><ol><li> <span class="w">Left</span>      <span class="w">Right</span>      <span class="w">Description</span> <a class="l_k" href="functions/and.html">and</a> <span class="w">pseudocode</span></li><li> ===============================================================</li><li> <span class="w">Any</span>       <a class="l_k" href="functions/undef.html">undef</a>      <span class="w">check</span> <span class="w">whether</span> <span class="w">Any</span> <span class="w">is</span> <span class="w">undefined</span></li><li>                <span class="w">like</span><span class="co">:</span> !<a class="l_k" href="functions/defined.html">defined</a> <span class="w">Any</span></li><li></li><li> <span class="w">Any</span>       <span class="w">Object</span>     <span class="w">invoke</span> ~~ <span class="w">overloading</span> <span class="w">on</span> <span class="w">Object</span><span class="cm">,</span> <a class="l_k" href="functions/or.html">or</a> <a class="l_k" href="functions/die.html">die</a></li><li></li><li> <span class="w">Right</span> <span class="w">operand</span> <span class="w">is</span> <span class="w">an</span> <span class="w">ARRAY</span><span class="co">:</span></li><li></li><li> <span class="w">Left</span>      <span class="w">Right</span>      <span class="w">Description</span> <a class="l_k" href="functions/and.html">and</a> <span class="w">pseudocode</span></li><li> ===============================================================</li><li> <span class="w">ARRAY1</span>    <span class="w">ARRAY2</span>     <span class="w">recurse</span> <span class="w">on</span> <span class="w">paired</span> <span class="w">elements</span> <span class="w">of</span> <span class="w">ARRAY1</span> <a class="l_k" href="functions/and.html">and</a> <span class="w">ARRAY2</span><span class="s">[</span><span class="n">2</span><span class="s">]</span></li><li>                <span class="w">like</span><span class="co">:</span> <span class="s">(</span><span class="w">ARRAY1</span><span class="s">[</span><span class="n">0</span><span class="s">]</span> ~~ <span class="w">ARRAY2</span><span class="s">[</span><span class="n">0</span><span class="s">]</span><span class="s">)</span></li><li>                        &amp;&amp; <span class="s">(</span><span class="w">ARRAY1</span><span class="s">[</span><span class="n">1</span><span class="s">]</span> ~~ <span class="w">ARRAY2</span><span class="s">[</span><span class="n">1</span><span class="s">]</span><span class="s">)</span> &amp;&amp; ...</li><li> <span class="w">HASH</span>      <span class="w">ARRAY</span>      <span class="w">any</span> <span class="w">ARRAY</span> <span class="w">elements</span> <span class="w">exist</span> <span class="w">as</span> <span class="w">HASH</span> <a class="l_k" href="functions/keys.html">keys</a></li><li>                <span class="w">like</span><span class="co">:</span> <a class="l_k" href="functions/grep.html">grep</a> <span class="s">{</span> <a class="l_k" href="functions/exists.html">exists</a> <span class="w">HASH</span>-&gt;{<span class="i">$_</span>} <span class="s">}</span> <span class="w">ARRAY</span></li><li> <span class="w">Regexp</span>    <span class="w">ARRAY</span>      <span class="w">any</span> <span class="w">ARRAY</span> <span class="w">elements</span> <span class="w">pattern</span> <span class="w">match</span> <span class="w">Regexp</span></li><li>                <span class="w">like</span><span class="co">:</span> <a class="l_k" href="functions/grep.html">grep</a> <span class="s">{</span> <span class="q">/Regexp/</span> <span class="s">}</span> <span class="w">ARRAY</span></li><li> <a class="l_k" href="functions/undef.html">undef</a>     <span class="w">ARRAY</span>      <a class="l_k" href="functions/undef.html">undef</a> <span class="w">in</span> <span class="w">ARRAY</span></li><li>                <span class="w">like</span><span class="co">:</span> <a class="l_k" href="functions/grep.html">grep</a> <span class="s">{</span> !<a class="l_k" href="functions/defined.html">defined</a> <span class="s">}</span> <span class="w">ARRAY</span></li><li> <span class="w">Any</span>       <span class="w">ARRAY</span>      <span class="w">smartmatch</span> <a class="l_k" href="functions/each.html">each</a> <span class="w">ARRAY</span> <span class="w">element</span><span class="s">[</span><span class="n">3</span><span class="s">]</span></li><li>                <span class="w">like</span><span class="co">:</span> <a class="l_k" href="functions/grep.html">grep</a> <span class="s">{</span> <span class="w">Any</span> ~~ <span class="i">$_</span> <span class="s">}</span> <span class="w">ARRAY</span></li><li></li><li> <span class="w">Right</span> <span class="w">operand</span> <span class="w">is</span> <span class="w">a</span> <span class="w">HASH</span><span class="co">:</span></li><li></li><li> <span class="w">Left</span>      <span class="w">Right</span>      <span class="w">Description</span> <a class="l_k" href="functions/and.html">and</a> <span class="w">pseudocode</span></li><li> ===============================================================</li><li> <span class="w">HASH1</span>     <span class="w">HASH2</span>      <span class="w">all</span> <span class="w">same</span> <a class="l_k" href="functions/keys.html">keys</a> <span class="w">in</span> <span class="w">both</span> <span class="w">HASHes</span></li><li>                <span class="w">like</span><span class="co">:</span> <a class="l_k" href="functions/keys.html">keys</a> <span class="w">HASH1</span> ==</li><li>                         <a class="l_k" href="functions/grep.html">grep</a> <span class="s">{</span> <a class="l_k" href="functions/exists.html">exists</a> <span class="w">HASH2</span>-&gt;{<span class="i">$_</span>} <span class="s">}</span> <a class="l_k" href="functions/keys.html">keys</a> <span class="w">HASH1</span></li><li> <span class="w">ARRAY</span>     <span class="w">HASH</span>       <span class="w">any</span> <span class="w">ARRAY</span> <span class="w">elements</span> <span class="w">exist</span> <span class="w">as</span> <span class="w">HASH</span> <a class="l_k" href="functions/keys.html">keys</a></li><li>                <span class="w">like</span><span class="co">:</span> <a class="l_k" href="functions/grep.html">grep</a> <span class="s">{</span> <a class="l_k" href="functions/exists.html">exists</a> <span class="w">HASH</span>-&gt;{<span class="i">$_</span>} <span class="s">}</span> <span class="w">ARRAY</span></li><li> <span class="w">Regexp</span>    <span class="w">HASH</span>       <span class="w">any</span> <span class="w">HASH</span> <a class="l_k" href="functions/keys.html">keys</a> <span class="w">pattern</span> <span class="w">match</span> <span class="w">Regexp</span></li><li>                <span class="w">like</span><span class="co">:</span> <a class="l_k" href="functions/grep.html">grep</a> <span class="s">{</span> <span class="q">/Regexp/</span> <span class="s">}</span> <a class="l_k" href="functions/keys.html">keys</a> <span class="w">HASH</span></li><li> <a class="l_k" href="functions/undef.html">undef</a>     <span class="w">HASH</span>       <span class="w">always</span> <span class="w">false</span> <span class="s">(</span><a class="l_k" href="functions/undef.html">undef</a> <span class="w">can&#39;t</span> <span class="w">be</span> <span class="w">a</span> <span class="w">key</span><span class="s">)</span></li><li>                <span class="w">like</span><span class="co">:</span> <span class="n">0</span> == <span class="n">1</span></li><li> <span class="w">Any</span>       <span class="w">HASH</span>       <span class="w">HASH</span> <span class="w">key</span> <span class="w">existence</span></li><li>                <span class="w">like</span><span class="co">:</span> <a class="l_k" href="functions/exists.html">exists</a> <span class="w">HASH</span>-&gt;{<span class="w">Any</span>}</li><li></li><li> <span class="w">Right</span> <span class="w">operand</span> <span class="w">is</span> <span class="w">CODE</span><span class="co">:</span></li><li></li><li> <span class="w">Left</span>      <span class="w">Right</span>      <span class="w">Description</span> <a class="l_k" href="functions/and.html">and</a> <span class="w">pseudocode</span></li><li> ===============================================================</li><li><a name="returns"></a> <span class="w">ARRAY</span>     <span class="w">CODE</span>       sub <span class="m">returns</span> <span class="w">true</span> <span class="w">on</span> <span class="w">all</span> <span class="w">ARRAY</span> <span class="w">elements</span><span class="s">[</span><span class="n">1</span><span class="s">]</span></li><li>                <span class="w">like</span><span class="co">:</span> !<a class="l_k" href="functions/grep.html">grep</a> <span class="s">{</span> !<span class="w">CODE</span>-&gt;<span class="s">(</span><span class="i">$_</span><span class="s">)</span> <span class="s">}</span> <span class="w">ARRAY</span></li><li><a name="returns-1"></a> <span class="w">HASH</span>      <span class="w">CODE</span>       sub <span class="m">returns</span> <span class="w">true</span> <span class="w">on</span> <span class="w">all</span> <span class="w">HASH</span> <a class="l_k" href="functions/keys.html">keys</a><span class="s">[</span><span class="n">1</span><span class="s">]</span></li><li>                <span class="w">like</span><span class="co">:</span> !<a class="l_k" href="functions/grep.html">grep</a> <span class="s">{</span> !<span class="w">CODE</span>-&gt;<span class="s">(</span><span class="i">$_</span><span class="s">)</span> <span class="s">}</span> <a class="l_k" href="functions/keys.html">keys</a> <span class="w">HASH</span></li><li><a name="passed"></a> <span class="w">Any</span>       <span class="w">CODE</span>       sub <span class="m">passed</span> <span class="w">Any</span> <span class="i">returns</span> <span class="w">true</span></li><li>                <span class="w">like</span><span class="co">:</span> <span class="w">CODE</span>-&gt;<span class="s">(</span><span class="w">Any</span><span class="s">)</span></li></ol></pre><p>Right operand is a Regexp:</p>
<pre class="verbatim"><ol><li> <span class="w">Left</span>      <span class="w">Right</span>      <span class="w">Description</span> <a class="l_k" href="functions/and.html">and</a> <span class="w">pseudocode</span></li><li> ===============================================================</li><li> <span class="w">ARRAY</span>     <span class="w">Regexp</span>     <span class="w">any</span> <span class="w">ARRAY</span> <span class="w">elements</span> <span class="w">match</span> <span class="w">Regexp</span></li><li>                <span class="w">like</span><span class="co">:</span> <a class="l_k" href="functions/grep.html">grep</a> <span class="s">{</span> <span class="q">/Regexp/</span> <span class="s">}</span> <span class="w">ARRAY</span></li><li> <span class="w">HASH</span>      <span class="w">Regexp</span>     <span class="w">any</span> <span class="w">HASH</span> <a class="l_k" href="functions/keys.html">keys</a> <span class="w">match</span> <span class="w">Regexp</span></li><li>                <span class="w">like</span><span class="co">:</span> <a class="l_k" href="functions/grep.html">grep</a> <span class="s">{</span> <span class="q">/Regexp/</span> <span class="s">}</span> <a class="l_k" href="functions/keys.html">keys</a> <span class="w">HASH</span></li><li> <span class="w">Any</span>       <span class="w">Regexp</span>     <span class="w">pattern</span> <span class="w">match</span></li><li>                <span class="w">like</span><span class="co">:</span> <span class="w">Any</span> =~ <span class="q">/Regexp/</span></li><li></li><li> <span class="w">Other</span><span class="co">:</span></li><li></li><li> <span class="w">Left</span>      <span class="w">Right</span>      <span class="w">Description</span> <a class="l_k" href="functions/and.html">and</a> <span class="w">pseudocode</span></li><li> ===============================================================</li><li> <span class="w">Object</span>    <span class="w">Any</span>        <span class="w">invoke</span> ~~ <span class="w">overloading</span> <span class="w">on</span> <span class="w">Object</span><span class="cm">,</span></li><li>                      <a class="l_k" href="functions/or.html">or</a> <span class="w">fall</span> <span class="w">back</span> <span class="w">to</span>...</li><li></li><li> <span class="w">Any</span>       <span class="w">Num</span>        <span class="w">numeric</span> <span class="w">equality</span></li><li>                 <span class="w">like</span><span class="co">:</span> <span class="w">Any</span> == <span class="w">Num</span></li><li> <span class="w">Num</span>       <span class="w">nummy</span><span class="s">[</span><span class="n">4</span><span class="s">]</span>    <span class="w">numeric</span> <span class="w">equality</span></li><li>                 <span class="w">like</span><span class="co">:</span> <span class="w">Num</span> == <span class="w">nummy</span></li><li> <a class="l_k" href="functions/undef.html">undef</a>     <span class="w">Any</span>        <span class="w">check</span> <span class="w">whether</span> <span class="w">undefined</span></li><li>                 <span class="w">like</span><span class="co">:</span> !<a class="l_k" href="functions/defined.html">defined</a><span class="s">(</span><span class="w">Any</span><span class="s">)</span></li><li> <span class="w">Any</span>       <span class="w">Any</span>        <span class="w">string</span> <span class="w">equality</span></li><li>                 <span class="w">like</span><span class="co">:</span> <span class="w">Any</span> <a class="l_k" href="functions/eq.html">eq</a> <span class="w">Any</span></li></ol></pre><p>Notes:</p>
<dl>
<dt>1.
</dt><dd><a name="1.-Empty-hashes-or-arrays-match."></a><b>Empty hashes or arrays match.</b>
</dd>
<dt>2.
</dt><dd><a name="2.-That-is%2c-each-element-smartmatches-the-element-of-the-same-index-in-the-other-array.%5b3%5d"></a><b>That is, each element smartmatches the element of the same index in the other array.[3]</b>
</dd>
<dt>3.
</dt><dd><a name="3.-If-a-circular-reference-is-found%2c-fall-back-to-referential-equality."></a><b>If a circular reference is found, fall back to referential equality.</b>
</dd>
<dt>4.
</dt><dd><a name="4.-Either-an-actual-number%2c-or-a-string-that-looks-like-one."></a><b>Either an actual number, or a string that looks like one.</b>
</dd>
</dl>
<p>The smartmatch implicitly dereferences any non-blessed hash or array
reference, so the <code class="inline"><i>HASH</i></code> and <code class="inline"><i>ARRAY</i></code> entries apply in those cases.
For blessed references, the <code class="inline"><i>Object</i></code> entries apply.  Smartmatches
involving hashes only consider hash keys, never hash values.</p>
<p>The "like" code entry is not always an exact rendition.  For example, the
smartmatch operator short-circuits whenever possible, but <code class="inline"><a class="l_k" href="functions/grep.html">grep</a></code> does
not.  Also, <code class="inline"><a class="l_k" href="functions/grep.html">grep</a></code> in scalar context returns the number of matches, but
<code class="inline">~~</code>
 returns only true or false.</p>
<p>Unlike most operators, the smartmatch operator knows to treat <code class="inline"><a class="l_k" href="functions/undef.html">undef</a></code>
specially:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/use.html">use</a> <span class="v">v5.10.1</span><span class="sc">;</span></li><li>    <span class="i">@array</span> = <span class="s">(</span><span class="n">1</span><span class="cm">,</span> <span class="n">2</span><span class="cm">,</span> <span class="n">3</span><span class="cm">,</span> <a class="l_k" href="functions/undef.html">undef</a><span class="cm">,</span> <span class="n">4</span><span class="cm">,</span> <span class="n">5</span><span class="s">)</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/say.html">say</a> <span class="q">&quot;some elements undefined&quot;</span> <a class="l_k" href="functions/if.html">if</a> <a class="l_k" href="functions/undef.html">undef</a> ~~ <span class="i">@array</span><span class="sc">;</span></li></ol></pre><p>Each operand is considered in a modified scalar context, the modification
being that array and hash variables are passed by reference to the
operator, which implicitly dereferences them.  Both elements
of each pair are the same:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/use.html">use</a> <span class="v">v5.10.1</span><span class="sc">;</span></li><li></li><li>    <a class="l_k" href="functions/my.html">my</a> <span class="i">%hash</span> = <span class="s">(</span><span class="w">red</span>    <span class="cm">=&gt;</span> <span class="n">1</span><span class="cm">,</span> <span class="w">blue</span>   <span class="cm">=&gt;</span> <span class="n">2</span><span class="cm">,</span> <span class="w">green</span>  <span class="cm">=&gt;</span> <span class="n">3</span><span class="cm">,</span></li><li>                <span class="w">orange</span> <span class="cm">=&gt;</span> <span class="n">4</span><span class="cm">,</span> <span class="w">yellow</span> <span class="cm">=&gt;</span> <span class="n">5</span><span class="cm">,</span> <span class="w">purple</span> <span class="cm">=&gt;</span> <span class="n">6</span><span class="cm">,</span></li><li>                <span class="w">black</span>  <span class="cm">=&gt;</span> <span class="n">7</span><span class="cm">,</span> <span class="w">grey</span>   <span class="cm">=&gt;</span> <span class="n">8</span><span class="cm">,</span> <span class="w">white</span>  <span class="cm">=&gt;</span> <span class="n">9</span><span class="s">)</span><span class="sc">;</span></li><li></li><li>    <a class="l_k" href="functions/my.html">my</a> <span class="i">@array</span> = <span class="q">qw(red blue green)</span><span class="sc">;</span></li><li></li><li>    <a class="l_k" href="functions/say.html">say</a> <span class="q">&quot;some array elements in hash keys&quot;</span> <a class="l_k" href="functions/if.html">if</a>  <span class="i">@array</span> ~~  <span class="i">%hash</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/say.html">say</a> <span class="q">&quot;some array elements in hash keys&quot;</span> <a class="l_k" href="functions/if.html">if</a> \<span class="i">@array</span> ~~ \<span class="i">%hash</span><span class="sc">;</span></li><li></li><li>    <a class="l_k" href="functions/say.html">say</a> <span class="q">&quot;red in array&quot;</span> <a class="l_k" href="functions/if.html">if</a> <span class="q">&quot;red&quot;</span> ~~  <span class="i">@array</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/say.html">say</a> <span class="q">&quot;red in array&quot;</span> <a class="l_k" href="functions/if.html">if</a> <span class="q">&quot;red&quot;</span> ~~ \<span class="i">@array</span><span class="sc">;</span></li><li></li><li>    <a class="l_k" href="functions/say.html">say</a> <span class="q">&quot;some keys end in e&quot;</span> <a class="l_k" href="functions/if.html">if</a> <span class="q">/e$/</span> ~~  <span class="i">%hash</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/say.html">say</a> <span class="q">&quot;some keys end in e&quot;</span> <a class="l_k" href="functions/if.html">if</a> <span class="q">/e$/</span> ~~ \<span class="i">%hash</span><span class="sc">;</span></li></ol></pre><p>Two arrays smartmatch if each element in the first array smartmatches
(that is, is "in") the corresponding element in the second array,
recursively.</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/use.html">use</a> <span class="v">v5.10.1</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/my.html">my</a> <span class="i">@little</span> = <span class="q">qw(red blue green)</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/my.html">my</a> <span class="i">@bigger</span> = <span class="s">(</span><span class="q">&quot;red&quot;</span><span class="cm">,</span> <span class="q">&quot;blue&quot;</span><span class="cm">,</span> <span class="s">[</span> <span class="q">&quot;orange&quot;</span><span class="cm">,</span> <span class="q">&quot;green&quot;</span> <span class="s">]</span> <span class="s">)</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/if.html">if</a> <span class="s">(</span><span class="i">@little</span> ~~ <span class="i">@bigger</span><span class="s">)</span> <span class="s">{</span>  <span class="c"># true!</span></li><li>        <a class="l_k" href="functions/say.html">say</a> <span class="q">&quot;little is contained in bigger&quot;</span><span class="sc">;</span></li><li>    <span class="s">}</span></li></ol></pre><p>Because the smartmatch operator recurses on nested arrays, this
will still report that "red" is in the array.</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/use.html">use</a> <span class="v">v5.10.1</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/my.html">my</a> <span class="i">@array</span> = <span class="q">qw(red blue green)</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/my.html">my</a> <span class="i">$nested_array</span> = <span class="s">[</span><span class="s">[</span><span class="s">[</span><span class="s">[</span><span class="s">[</span><span class="s">[</span><span class="s">[</span> <span class="i">@array</span> <span class="s">]</span><span class="s">]</span><span class="s">]</span><span class="s">]</span><span class="s">]</span><span class="s">]</span><span class="s">]</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/say.html">say</a> <span class="q">&quot;red in array&quot;</span> <a class="l_k" href="functions/if.html">if</a> <span class="q">&quot;red&quot;</span> ~~ <span class="i">$nested_array</span><span class="sc">;</span></li></ol></pre><p>If two arrays smartmatch each other, then they are deep
copies of each others' values, as this example reports:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/use.html">use</a> <span class="v">v5.12.0</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/my.html">my</a> <span class="i">@a</span> = <span class="s">(</span><span class="n">0</span><span class="cm">,</span> <span class="n">1</span><span class="cm">,</span> <span class="n">2</span><span class="cm">,</span> <span class="s">[</span><span class="n">3</span><span class="cm">,</span> <span class="s">[</span><span class="n">4</span><span class="cm">,</span> <span class="n">5</span><span class="s">]</span><span class="cm">,</span> <span class="n">6</span><span class="s">]</span><span class="cm">,</span> <span class="n">7</span><span class="s">)</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/my.html">my</a> <span class="i">@b</span> = <span class="s">(</span><span class="n">0</span><span class="cm">,</span> <span class="n">1</span><span class="cm">,</span> <span class="n">2</span><span class="cm">,</span> <span class="s">[</span><span class="n">3</span><span class="cm">,</span> <span class="s">[</span><span class="n">4</span><span class="cm">,</span> <span class="n">5</span><span class="s">]</span><span class="cm">,</span> <span class="n">6</span><span class="s">]</span><span class="cm">,</span> <span class="n">7</span><span class="s">)</span><span class="sc">;</span></li><li></li><li>    <a class="l_k" href="functions/if.html">if</a> <span class="s">(</span><span class="i">@a</span> ~~ <span class="i">@b</span> &amp;&amp; <span class="i">@b</span> ~~ <span class="i">@a</span><span class="s">)</span> <span class="s">{</span></li><li>        <a class="l_k" href="functions/say.html">say</a> <span class="q">&quot;a and b are deep copies of each other&quot;</span><span class="sc">;</span></li><li>    <span class="s">}</span></li><li>    <a class="l_k" href="functions/elsif.html">elsif</a> <span class="s">(</span><span class="i">@a</span> ~~ <span class="i">@b</span><span class="s">)</span> <span class="s">{</span></li><li>        <a class="l_k" href="functions/say.html">say</a> <span class="q">&quot;a smartmatches in b&quot;</span><span class="sc">;</span></li><li>    <span class="s">}</span></li><li>    <a class="l_k" href="functions/elsif.html">elsif</a> <span class="s">(</span><span class="i">@b</span> ~~ <span class="i">@a</span><span class="s">)</span> <span class="s">{</span></li><li>        <a class="l_k" href="functions/say.html">say</a> <span class="q">&quot;b smartmatches in a&quot;</span><span class="sc">;</span></li><li>    <span class="s">}</span></li><li>    <a class="l_k" href="functions/else.html">else</a> <span class="s">{</span></li><li>        <a class="l_k" href="functions/say.html">say</a> <span class="q">&quot;a and b don&#39;t smartmatch each other at all&quot;</span><span class="sc">;</span></li><li>    <span class="s">}</span></li></ol></pre><p>If you were to set <code class="inline"><span class="i">$b</span>[<span class="n">3</span>] = <span class="n">4</span></code>
, then instead of reporting that "a and b
are deep copies of each other", it now reports that <code class="inline"><span class="q">&quot;b smartmatches in a&quot;</span></code>
.
That's because the corresponding position in <code class="inline"><span class="i">@a</span></code>
 contains an array that
(eventually) has a 4 in it.</p>
<p>Smartmatching one hash against another reports whether both contain the
same keys, no more and no less.  This could be used to see whether two
records have the same field names, without caring what values those fields
might have.  For example:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/use.html">use</a> <span class="v">v5.10.1</span><span class="sc">;</span></li><li><a name="make_dogtag"></a>    sub <span class="m">make_dogtag</span> <span class="s">{</span></li><li>        <span class="w">state</span> <span class="i">$REQUIRED_FIELDS</span> = <span class="s">{</span> <span class="w">name</span><span class="cm">=&gt;</span><span class="n">1</span><span class="cm">,</span> <span class="w">rank</span><span class="cm">=&gt;</span><span class="n">1</span><span class="cm">,</span> <span class="w">serial_num</span><span class="cm">=&gt;</span><span class="n">1</span> <span class="s">}</span><span class="sc">;</span></li><li></li><li>        <a class="l_k" href="functions/my.html">my</a> <span class="s">(</span><span class="i">$class</span><span class="cm">,</span> <span class="i">$init_fields</span><span class="s">)</span> = <span class="i">@_</span><span class="sc">;</span></li><li></li><li>        <a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;Must supply (only) name, rank, and serial number&quot;</span></li><li>            <a class="l_k" href="functions/unless.html">unless</a> <span class="i">$init_fields</span> ~~ <span class="i">$REQUIRED_FIELDS</span><span class="sc">;</span></li><li></li><li>        ...</li><li>    <span class="s">}</span></li></ol></pre><p>However, this only does what you mean if <code class="inline"><span class="i">$init_fields</span></code>
 is indeed a hash
reference. The condition <code class="inline"><span class="i">$init_fields</span> ~~ <span class="i">$REQUIRED_FIELDS</span></code>
 also allows the
strings <code class="inline"><span class="q">&quot;name&quot;</span></code>
, <code class="inline"><span class="q">&quot;rank&quot;</span></code>
, <code class="inline"><span class="q">&quot;serial_num&quot;</span></code>
 as well as any array reference
that contains <code class="inline"><span class="q">&quot;name&quot;</span></code>
 or <code class="inline"><span class="q">&quot;rank&quot;</span></code>
 or <code class="inline"><span class="q">&quot;serial_num&quot;</span></code>
 anywhere to pass
through.</p>
<p>The smartmatch operator is most often used as the implicit operator of a
<code class="inline">when</code>
 clause.  See the section on "Switch Statements" in <a href="perlsyn.html">perlsyn</a>.</p>
<a name="Smartmatching-of-Objects"></a><h3>Smartmatching of Objects</h3>
<p>To avoid relying on an object's underlying representation, if the
smartmatch's right operand is an object that doesn't overload <code class="inline">~~</code>
,
it raises the exception "<code class="inline"><span class="w">Smartmatching</span> <span class="w">a</span> <span class="w">non</span>-<span class="w">overloaded</span> <span class="w">object</span>
<span class="w">breaks</span> <span class="w">encapsulation</span></code>
".  That's because one has no business digging
around to see whether something is "in" an object.  These are all
illegal on objects without a <code class="inline">~~</code>
 overload:</p>
<pre class="verbatim"><ol><li>    <span class="i">%hash</span> ~~ <span class="i">$object</span></li><li>       <span class="n">42</span> ~~ <span class="i">$object</span></li><li>   <span class="q">&quot;fred&quot;</span> ~~ <span class="i">$object</span></li></ol></pre><p>However, you can change the way an object is smartmatched by overloading
the <code class="inline">~~</code>
 operator.  This is allowed to
extend the usual smartmatch semantics.
For objects that do have an <code class="inline">~~</code>
 overload, see <a href="overload.html">overload</a>.</p>
<p>Using an object as the left operand is allowed, although not very useful.
Smartmatching rules take precedence over overloading, so even if the
object in the left operand has smartmatch overloading, this will be
ignored.  A left operand that is a non-overloaded object falls back on a
string or numeric comparison of whatever the <code class="inline"><a class="l_k" href="functions/ref.html">ref</a></code> operator returns.  That
means that</p>
<pre class="verbatim"><ol><li>    <span class="i">$object</span> ~~ <span class="w">X</span></li></ol></pre><p>does <i>not</i> invoke the overload method with <code class="inline"><i>X</i></code> as an argument.
Instead the above table is consulted as normal, and based on the type of
<code class="inline"><i>X</i></code>, overloading may or may not be invoked.  For simple strings or
numbers, "in" becomes equivalent to this:</p>
<pre class="verbatim"><ol><li>    <span class="i">$object</span> ~~ <span class="i">$number</span>          <a class="l_k" href="functions/ref.html">ref</a><span class="s">(</span><span class="i">$object</span><span class="s">)</span> == <span class="i">$number</span></li><li>    <span class="i">$object</span> ~~ <span class="i">$string</span>          <a class="l_k" href="functions/ref.html">ref</a><span class="s">(</span><span class="i">$object</span><span class="s">)</span> <a class="l_k" href="functions/eq.html">eq</a> <span class="i">$string</span></li></ol></pre><p>For example, this reports that the handle smells IOish
(but please don't really do this!):</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/use.html">use</a> <span class="w">IO::Handle</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/my.html">my</a> <span class="i">$fh</span> = <span class="w">IO::Handle</span><span class="w">-&gt;new</span><span class="s">(</span><span class="s">)</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/if.html">if</a> <span class="s">(</span><span class="i">$fh</span> ~~ <span class="q">/\bIO\b/</span><span class="s">)</span> <span class="s">{</span></li><li>        <a class="l_k" href="functions/say.html">say</a> <span class="q">&quot;handle smells IOish&quot;</span><span class="sc">;</span></li><li>    <span class="s">}</span></li></ol></pre><p>That's because it treats <code class="inline"><span class="i">$fh</span></code>
 as a string like
<code class="inline"><span class="q">&quot;IO::Handle=GLOB(0x8039e0)&quot;</span></code>
, then pattern matches against that.</p>
<a name="Bitwise-And"></a><h2>Bitwise And
  </h2>
<p>Binary <code class="inline"><span class="q">&quot;&amp;&quot;</span></code>
 returns its operands ANDed together bit by bit.  Although no
warning is currently raised, the result is not well defined when this operation
is performed on operands that aren't either numbers (see
<a href="#Integer-Arithmetic">Integer Arithmetic</a>) nor bitstrings (see <a href="#Bitwise-String-Operators">Bitwise String Operators</a>).</p>
<p>Note that <code class="inline"><span class="q">&quot;&amp;&quot;</span></code>
 has lower priority than relational operators, so for example
the parentheses are essential in a test like</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;Even\n&quot;</span> <a class="l_k" href="functions/if.html">if</a> <span class="s">(</span><span class="i">$x</span> &amp; <span class="n">1</span><span class="s">)</span> == <span class="n">0</span><span class="sc">;</span></li></ol></pre><p>If the experimental "bitwise" feature is enabled via <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">feature</span>
<span class="q">&#39;bitwise&#39;</span></code>
, then this operator always treats its operand as numbers.  This
feature produces a warning unless you also use <code class="inline"><a class="l_k" href="functions/no.html">no</a> <span class="w">warnings</span>
<span class="q">&#39;experimental::bitwise&#39;</span></code>
.</p>
<a name="Bitwise-Or-and-Exclusive-Or"></a><h2>Bitwise Or and Exclusive Or
   
 </h2>
<p>Binary <code class="inline"><span class="q">&quot;|&quot;</span></code>
 returns its operands ORed together bit by bit.</p>
<p>Binary <code class="inline"><span class="q">&quot;^&quot;</span></code>
 returns its operands XORed together bit by bit.</p>
<p>Although no warning is currently raised, the results are not well
defined when these operations are performed on operands that aren't either
numbers (see <a href="#Integer-Arithmetic">Integer Arithmetic</a>) nor bitstrings (see <a href="#Bitwise-String-Operators">Bitwise String Operators</a>).</p>
<p>Note that <code class="inline"><span class="q">&quot;|&quot;</span></code>
 and <code class="inline"><span class="q">&quot;^&quot;</span></code>
 have lower priority than relational operators, so
for example the parentheses are essential in a test like</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;false\n&quot;</span> <a class="l_k" href="functions/if.html">if</a> <span class="s">(</span><span class="n">8</span> | <span class="n">2</span><span class="s">)</span> != <span class="n">10</span><span class="sc">;</span></li></ol></pre><p>If the experimental "bitwise" feature is enabled via <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">feature</span>
<span class="q">&#39;bitwise&#39;</span></code>
, then this operator always treats its operand as numbers.  This
feature produces a warning unless you also use <code class="inline"><a class="l_k" href="functions/no.html">no</a> <span class="w">warnings</span>
<span class="q">&#39;experimental::bitwise&#39;</span></code>
.</p>
<a name="C-style-Logical-And"></a><h2>C-style Logical And
  </h2>
<p>Binary <code class="inline"><span class="q">&quot;&amp;&amp;&quot;</span></code>
 performs a short-circuit logical AND operation.  That is,
if the left operand is false, the right operand is not even evaluated.
Scalar or list context propagates down to the right operand if it
is evaluated.</p>
<a name="C-style-Logical-Or"></a><h2>C-style Logical Or
 </h2>
<p>Binary <code class="inline"><span class="q">&quot;||&quot;</span></code>
 performs a short-circuit logical OR operation.  That is,
if the left operand is true, the right operand is not even evaluated.
Scalar or list context propagates down to the right operand if it
is evaluated.</p>
<a name="Logical-Defined-Or"></a><h2>Logical Defined-Or
 </h2>
<p>Although it has no direct equivalent in C, Perl's <code class="inline"><span class="q">//</span></code>
 operator is related
to its C-style "or".  In fact, it's exactly the same as <code class="inline">||</code>, except that it
tests the left hand side's definedness instead of its truth.  Thus,
<code class="inline"><span class="w">EXPR1</span> <span class="q">//</span> <span class="w">EXPR2</span></code>
 returns the value of <code class="inline"><span class="w">EXPR1</span></code>
 if it's defined,
otherwise, the value of <code class="inline"><span class="w">EXPR2</span></code>
 is returned.
(<code class="inline"><span class="w">EXPR1</span></code>
 is evaluated in scalar context, <code class="inline"><span class="w">EXPR2</span></code>

in the context of <code class="inline"><span class="q">//</span></code>
 itself).  Usually,
this is the same result as <code class="inline"><a class="l_k" href="functions/defined.html">defined</a><span class="s">(</span><span class="w">EXPR1</span><span class="s">)</span> ? <span class="w">EXPR1</span> <span class="co">:</span> <span class="w">EXPR2</span></code>
 (except that
the ternary-operator form can be used as a lvalue, while <code class="inline"><span class="w">EXPR1</span> <span class="q">//</span> <span class="w">EXPR2</span></code>

cannot).  This is very useful for
providing default values for variables.  If you actually want to test if
at least one of <code class="inline"><span class="i">$x</span></code>
 and <code class="inline"><span class="i">$y</span></code>
 is defined, use <code class="inline"><a class="l_k" href="functions/defined.html">defined</a><span class="s">(</span><span class="i">$x</span> // <span class="i">$y</span><span class="s">)</span></code>
.</p>
<p>The <code class="inline">||</code>, <code class="inline"><span class="q">//</span></code>
 and <code class="inline">&amp;&amp;</code> operators return the last value evaluated
(unlike C's <code class="inline">||</code> and <code class="inline">&amp;&amp;</code>, which return 0 or 1).  Thus, a reasonably
portable way to find out the home directory might be:</p>
<pre class="verbatim"><ol><li>    <span class="i">$home</span> =  <span class="i">$ENV</span>{<span class="w">HOME</span>}</li><li>	  // <span class="i">$ENV</span>{<span class="w">LOGDIR</span>}</li><li>	  // <span class="s">(</span><a class="l_k" href="functions/getpwuid.html">getpwuid</a><span class="s">(</span><span class="i">$&lt;</span><span class="s">)</span><span class="s">)</span>[<span class="n">7</span>]</li><li>	  // <a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;You&#39;re homeless!\n&quot;</span><span class="sc">;</span></li></ol></pre><p>In particular, this means that you shouldn't use this
for selecting between two aggregates for assignment:</p>
<pre class="verbatim"><ol><li>    <span class="i">@a</span> = <span class="i">@b</span> || <span class="i">@c</span><span class="sc">;</span>            <span class="c"># This doesn&#39;t do the right thing</span></li><li>    <span class="i">@a</span> = <a class="l_k" href="functions/scalar.html">scalar</a><span class="s">(</span><span class="i">@b</span><span class="s">)</span> || <span class="i">@c</span><span class="sc">;</span>    <span class="c"># because it really means this.</span></li><li>    <span class="i">@a</span> = <span class="i">@b</span> ? <span class="i">@b</span> <span class="co">:</span> <span class="i">@c</span><span class="sc">;</span>        <span class="c"># This works fine, though.</span></li></ol></pre><p>As alternatives to <code class="inline">&amp;&amp;</code> and <code class="inline">||</code> when used for
control flow, Perl provides the <code class="inline">and</code>
 and <code class="inline">or</code>
 operators (see below).
The short-circuit behavior is identical.  The precedence of <code class="inline"><span class="q">&quot;and&quot;</span></code>

and <code class="inline"><span class="q">&quot;or&quot;</span></code>
 is much lower, however, so that you can safely use them after a
list operator without the need for parentheses:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/unlink.html">unlink</a> <span class="q">&quot;alpha&quot;</span><span class="cm">,</span> <span class="q">&quot;beta&quot;</span><span class="cm">,</span> <span class="q">&quot;gamma&quot;</span></li><li>	    or <span class="i">gripe</span><span class="s">(</span><span class="s">)</span><span class="cm">,</span> <a class="l_k" href="functions/next.html">next</a> <span class="j">LINE</span><span class="sc">;</span></li></ol></pre><p>With the C-style operators that would have been written like this:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/unlink.html">unlink</a><span class="s">(</span><span class="q">&quot;alpha&quot;</span><span class="cm">,</span> <span class="q">&quot;beta&quot;</span><span class="cm">,</span> <span class="q">&quot;gamma&quot;</span><span class="s">)</span></li><li>	    || <span class="s">(</span><span class="i">gripe</span><span class="s">(</span><span class="s">)</span><span class="cm">,</span> <a class="l_k" href="functions/next.html">next</a> <span class="j">LINE</span><span class="s">)</span><span class="sc">;</span></li></ol></pre><p>It would be even more readable to write that this way:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/unless.html">unless</a><span class="s">(</span><a class="l_k" href="functions/unlink.html">unlink</a><span class="s">(</span><span class="q">&quot;alpha&quot;</span><span class="cm">,</span> <span class="q">&quot;beta&quot;</span><span class="cm">,</span> <span class="q">&quot;gamma&quot;</span><span class="s">)</span><span class="s">)</span> <span class="s">{</span></li><li>        <span class="i">gripe</span><span class="s">(</span><span class="s">)</span><span class="sc">;</span></li><li>        <a class="l_k" href="functions/next.html">next</a> <span class="j">LINE</span><span class="sc">;</span></li><li>    <span class="s">}</span></li></ol></pre><p>Using <code class="inline"><span class="q">&quot;or&quot;</span></code>
 for assignment is unlikely to do what you want; see below.</p>
<a name="Range-Operators"></a><h2>Range Operators
   </h2>
<p>Binary <code class="inline"><span class="q">&quot;..&quot;</span></code>
 is the range operator, which is really two different
operators depending on the context.  In list context, it returns a
list of values counting (up by ones) from the left value to the right
value.  If the left value is greater than the right value then it
returns the empty list.  The range operator is useful for writing
<code class="inline">foreach <span class="s">(</span><span class="n">1</span>..<span class="n">10</span><span class="s">)</span></code>
 loops and for doing slice operations on arrays.  In
the current implementation, no temporary array is created when the
range operator is used as the expression in <code class="inline">foreach</code>
 loops, but older
versions of Perl might burn a lot of memory when you write something
like this:</p>
<pre class="verbatim"><ol><li>    for <span class="s">(</span><span class="n">1</span> .. <span class="n">1_000_000</span><span class="s">)</span> <span class="s">{</span></li><li>	<span class="c"># code</span></li><li>    <span class="s">}</span></li></ol></pre><p>The range operator also works on strings, using the magical
auto-increment, see below.</p>
<p>In scalar context, <code class="inline"><span class="q">&quot;..&quot;</span></code>
 returns a boolean value.  The operator is
bistable, like a flip-flop, and emulates the line-range (comma)
operator of <b>sed</b>, <b>awk</b>, and various editors.  Each <code class="inline"><span class="q">&quot;..&quot;</span></code>
 operator
maintains its own boolean state, even across calls to a subroutine
that contains it.  It is false as long as its left operand is false.
Once the left operand is true, the range operator stays true until the
right operand is true, <i>AFTER</i> which the range operator becomes false
again.  It doesn't become false till the next time the range operator
is evaluated.  It can test the right operand and become false on the
same evaluation it became true (as in <b>awk</b>), but it still returns
true once.  If you don't want it to test the right operand until the
next evaluation, as in <b>sed</b>, just use three dots (<code class="inline"><span class="q">&quot;...&quot;</span></code>
) instead of
two.  In all other regards, <code class="inline"><span class="q">&quot;...&quot;</span></code>
 behaves just like <code class="inline"><span class="q">&quot;..&quot;</span></code>
 does.</p>
<p>The right operand is not evaluated while the operator is in the
"false" state, and the left operand is not evaluated while the
operator is in the "true" state.  The precedence is a little lower
than || and &amp;&amp;.  The value returned is either the empty string for
false, or a sequence number (beginning with 1) for true.  The sequence
number is reset for each range encountered.  The final sequence number
in a range has the string <code class="inline"><span class="q">&quot;E0&quot;</span></code>
 appended to it, which doesn't affect
its numeric value, but gives you something to search for if you want
to exclude the endpoint.  You can exclude the beginning point by
waiting for the sequence number to be greater than 1.</p>
<p>If either operand of scalar <code class="inline"><span class="q">&quot;..&quot;</span></code>
 is a constant expression,
that operand is considered true if it is equal (<code class="inline">==</code>
) to the current
input line number (the <code class="inline"><span class="i">$.</span></code>
 variable).</p>
<p>To be pedantic, the comparison is actually <code class="inline"><a class="l_k" href="functions/int.html">int</a><span class="s">(</span><span class="w">EXPR</span><span class="s">)</span> == <a class="l_k" href="functions/int.html">int</a><span class="s">(</span><span class="w">EXPR</span><span class="s">)</span></code>
,
but that is only an issue if you use a floating point expression; when
implicitly using <code class="inline"><span class="i">$.</span></code>
 as described in the previous paragraph, the
comparison is <code class="inline"><a class="l_k" href="functions/int.html">int</a><span class="s">(</span><span class="w">EXPR</span><span class="s">)</span> == <a class="l_k" href="functions/int.html">int</a><span class="s">(</span><span class="i">$.</span><span class="s">)</span></code>
 which is only an issue when <code class="inline"><span class="i">$.</span></code>

is set to a floating point value and you are not reading from a file.
Furthermore, <code class="inline"><span class="q">&quot;span&quot;</span> .. <span class="q">&quot;spat&quot;</span></code>
 or <code class="inline"><span class="n">2.18</span> .. <span class="n">3.14</span></code>
 will not do what
you want in scalar context because each of the operands are evaluated
using their integer representation.</p>
<p>Examples:</p>
<p>As a scalar operator:</p>
<pre class="verbatim"><ol><li>    if <span class="s">(</span><span class="n">101</span> .. <span class="n">200</span><span class="s">)</span> <span class="s">{</span> <a class="l_k" href="functions/print.html">print</a><span class="sc">;</span> <span class="s">}</span> <span class="c"># print 2nd hundred lines, short for</span></li><li>                               <span class="c">#  if ($. == 101 .. $. == 200) { print; }</span></li><li></li><li>    <a class="l_k" href="functions/next.html">next</a> <span class="j">LINE</span> if <span class="s">(</span><span class="n">1</span> .. <span class="q">/^$/</span><span class="s">)</span><span class="sc">;</span>  <span class="c"># skip header lines, short for</span></li><li>                               <span class="c">#   next LINE if ($. == 1 .. /^$/);</span></li><li>                               <span class="c"># (typically in a loop labeled LINE)</span></li><li></li><li>    <span class="q">s/^/&gt; /</span> if <span class="s">(</span><span class="q">/^$/</span> .. <a class="l_k" href="functions/eof.html">eof</a><span class="s">(</span><span class="s">)</span><span class="s">)</span><span class="sc">;</span>  <span class="c"># quote body</span></li><li></li><li>    <span class="c"># parse mail messages</span></li><li>    while <span class="s">(</span>&lt;&gt;<span class="s">)</span> <span class="s">{</span></li><li>        <span class="i">$in_header</span> =   <span class="n">1</span>  .. <span class="q">/^$/</span><span class="sc">;</span></li><li>        <span class="i">$in_body</span>   = <span class="q">/^$/</span> .. <a class="l_k" href="functions/eof.html">eof</a><span class="sc">;</span></li><li>        if <span class="s">(</span><span class="i">$in_header</span><span class="s">)</span> <span class="s">{</span></li><li>            <span class="c"># do something</span></li><li>        <span class="s">}</span> else <span class="s">{</span> <span class="c"># in body</span></li><li>            <span class="c"># do something else</span></li><li>        <span class="s">}</span></li><li>    <span class="s">}</span> <a class="l_k" href="functions/continue.html">continue</a> <span class="s">{</span></li><li>        <a class="l_k" href="functions/close.html">close</a> <span class="w">ARGV</span> if <a class="l_k" href="functions/eof.html">eof</a><span class="sc">;</span>             <span class="c"># reset $. each file</span></li><li>    <span class="s">}</span></li></ol></pre><p>Here's a simple example to illustrate the difference between
the two range operators:</p>
<pre class="verbatim"><ol><li>    <span class="i">@lines</span> = <span class="s">(</span><span class="q">&quot;   - Foo&quot;</span><span class="cm">,</span></li><li>              <span class="q">&quot;01 - Bar&quot;</span><span class="cm">,</span></li><li>              <span class="q">&quot;1  - Baz&quot;</span><span class="cm">,</span></li><li>              <span class="q">&quot;   - Quux&quot;</span><span class="s">)</span><span class="sc">;</span></li><li></li><li>    foreach <span class="s">(</span><span class="i">@lines</span><span class="s">)</span> <span class="s">{</span></li><li>        if <span class="s">(</span><span class="q">/0/</span> .. <span class="q">/1/</span><span class="s">)</span> <span class="s">{</span></li><li>            <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;$_\n&quot;</span><span class="sc">;</span></li><li>        <span class="s">}</span></li><li>    <span class="s">}</span></li></ol></pre><p>This program will print only the line containing "Bar".  If
the range operator is changed to <code class="inline">...</code>
, it will also print the
"Baz" line.</p>
<p>And now some examples as a list operator:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/for.html">for</a> <span class="s">(</span><span class="n">101</span> .. <span class="n">200</span><span class="s">)</span> <span class="s">{</span> <a class="l_k" href="functions/print.html">print</a> <span class="s">}</span>      <span class="c"># print $_ 100 times</span></li><li>    <span class="i">@foo</span> = <span class="i">@foo</span>[<span class="n">0</span> .. <span class="i">$#foo</span>]<span class="sc">;</span>        <span class="c"># an expensive no-op</span></li><li>    <span class="i">@foo</span> = <span class="i">@foo</span>[<span class="i">$#foo</span>-<span class="n">4</span> .. <span class="i">$#foo</span>]<span class="sc">;</span>  <span class="c"># slice last 5 items</span></li></ol></pre><p>The range operator (in list context) makes use of the magical
auto-increment algorithm if the operands are strings.  You
can say</p>
<pre class="verbatim"><ol><li>    <span class="i">@alphabet</span> = <span class="s">(</span><span class="q">&quot;A&quot;</span> .. <span class="q">&quot;Z&quot;</span><span class="s">)</span><span class="sc">;</span></li></ol></pre><p>to get all normal letters of the English alphabet, or</p>
<pre class="verbatim"><ol><li>    <span class="i">$hexdigit</span> = <span class="s">(</span><span class="n">0</span> .. <span class="n">9</span><span class="cm">,</span> <span class="q">&quot;a&quot;</span> .. <span class="q">&quot;f&quot;</span><span class="s">)</span>[<span class="i">$num</span> &amp; <span class="n">15</span>]<span class="sc">;</span></li></ol></pre><p>to get a hexadecimal digit, or</p>
<pre class="verbatim"><ol><li>    <span class="i">@z2</span> = <span class="s">(</span><span class="q">&quot;01&quot;</span> .. <span class="q">&quot;31&quot;</span><span class="s">)</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/print.html">print</a> <span class="i">$z2</span>[<span class="i">$mday</span>]<span class="sc">;</span></li></ol></pre><p>to get dates with leading zeros.</p>
<p>If the final value specified is not in the sequence that the magical
increment would produce, the sequence goes until the next value would
be longer than the final value specified.</p>
<p>As of Perl 5.26, the list-context range operator on strings works as expected
in the scope of <a href="feature.html#The-'unicode_strings'-feature">use feature &#39;unicode_strings </a>. In previous versions, and outside the scope of
that feature, it exhibits <a href="perlunicode.html#The-%22Unicode-Bug%22">The Unicode Bug in perlunicode</a>: its behavior
depends on the internal encoding of the range endpoint.</p>
<p>If the initial value specified isn't part of a magical increment
sequence (that is, a non-empty string matching <code class="inline"><span class="q">/^[a-zA-Z]*[0-9]*\z/</span></code>
),
only the initial value will be returned.  So the following will only
return an alpha:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/use.html">use</a> <span class="w">charnames</span> <span class="q">&quot;greek&quot;</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/my.html">my</a> <span class="i">@greek_small</span> =  <span class="s">(</span><span class="q">&quot;\N{alpha}&quot;</span> .. <span class="q">&quot;\N{omega}&quot;</span><span class="s">)</span><span class="sc">;</span></li></ol></pre><p>To get the 25 traditional lowercase Greek letters, including both sigmas,
you could use this instead:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/use.html">use</a> <span class="w">charnames</span> <span class="q">&quot;greek&quot;</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/my.html">my</a> <span class="i">@greek_small</span> =  <a class="l_k" href="functions/map.html">map</a> <span class="s">{</span> <a class="l_k" href="functions/chr.html">chr</a> <span class="s">}</span> <span class="s">(</span> <a class="l_k" href="functions/ord.html">ord</a><span class="s">(</span><span class="q">&quot;\N{alpha}&quot;</span><span class="s">)</span></li><li>                                        ..</li><li>                                     <a class="l_k" href="functions/ord.html">ord</a><span class="s">(</span><span class="q">&quot;\N{omega}&quot;</span><span class="s">)</span></li><li>                                   <span class="s">)</span><span class="sc">;</span></li></ol></pre><p>However, because there are <i>many</i> other lowercase Greek characters than
just those, to match lowercase Greek characters in a regular expression,
you could use the pattern <code class="inline"><span class="q">/(?:(?=\p{Greek})\p{Lower})+/</span></code>
 (or the
<a href="perlrecharclass.html#Extended-Bracketed-Character-Classes">experimental feature</a> <code class="inline"><span class="q">/(?[ \p{Greek} &amp; \p{Lower} ])+/</span></code>
).</p>
<p>Because each operand is evaluated in integer form, <code class="inline"><span class="n">2.18</span> .. <span class="n">3.14</span></code>
 will
return two elements in list context.</p>
<pre class="verbatim"><ol><li>    <span class="i">@list</span> = <span class="s">(</span><span class="n">2.18</span> .. <span class="n">3.14</span><span class="s">)</span><span class="sc">;</span> <span class="c"># same as @list = (2 .. 3);</span></li></ol></pre><a name="Conditional-Operator"></a><h2>Conditional Operator
   </h2>
<p>Ternary <code class="inline"><span class="q">&quot;?:&quot;</span></code>
 is the conditional operator, just as in C.  It works much
like an if-then-else.  If the argument before the <code class="inline">?</code> is true, the
argument before the <code class="inline"><span class="j">:</span></code>
 is returned, otherwise the argument after the
<code class="inline"><span class="j">:</span></code>
 is returned.  For example:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/printf.html">printf</a> <span class="q">&quot;I have %d dog%s.\n&quot;</span><span class="cm">,</span> <span class="i">$n</span><span class="cm">,</span></li><li>	    <span class="s">(</span><span class="i">$n</span> == <span class="n">1</span><span class="s">)</span> ? <span class="q">&quot;&quot;</span> <span class="co">:</span> <span class="q">&quot;s&quot;</span><span class="sc">;</span></li></ol></pre><p>Scalar or list context propagates downward into the 2nd
or 3rd argument, whichever is selected.</p>
<pre class="verbatim"><ol><li>    <span class="i">$x</span> = <span class="i">$ok</span> ? <span class="i">$y</span> <span class="co">:</span> <span class="i">$z</span><span class="sc">;</span>  <span class="c"># get a scalar</span></li><li>    <span class="i">@x</span> = <span class="i">$ok</span> ? <span class="i">@y</span> <span class="co">:</span> <span class="i">@z</span><span class="sc">;</span>  <span class="c"># get an array</span></li><li>    <span class="i">$x</span> = <span class="i">$ok</span> ? <span class="i">@y</span> <span class="co">:</span> <span class="i">@z</span><span class="sc">;</span>  <span class="c"># oops, that&#39;s just a count!</span></li></ol></pre><p>The operator may be assigned to if both the 2nd and 3rd arguments are
legal lvalues (meaning that you can assign to them):</p>
<pre class="verbatim"><ol><li>    <span class="s">(</span><span class="i">$x_or_y</span> ? <span class="i">$x</span> <span class="co">:</span> <span class="i">$y</span><span class="s">)</span> = <span class="i">$z</span><span class="sc">;</span></li></ol></pre><p>Because this operator produces an assignable result, using assignments
without parentheses will get you in trouble.  For example, this:</p>
<pre class="verbatim"><ol><li>    <span class="i">$x</span> % <span class="n">2</span> ? <span class="i">$x</span> += <span class="n">10</span> <span class="co">:</span> <span class="i">$x</span> += <span class="n">2</span></li></ol></pre><p>Really means this:</p>
<pre class="verbatim"><ol><li>    <span class="s">(</span><span class="s">(</span><span class="i">$x</span> % <span class="n">2</span><span class="s">)</span> ? <span class="s">(</span><span class="i">$x</span> += <span class="n">10</span><span class="s">)</span> <span class="co">:</span> <span class="i">$x</span><span class="s">)</span> += <span class="n">2</span></li></ol></pre><p>Rather than this:</p>
<pre class="verbatim"><ol><li>    <span class="s">(</span><span class="i">$x</span> % <span class="n">2</span><span class="s">)</span> ? <span class="s">(</span><span class="i">$x</span> += <span class="n">10</span><span class="s">)</span> <span class="co">:</span> <span class="s">(</span><span class="i">$x</span> += <span class="n">2</span><span class="s">)</span></li></ol></pre><p>That should probably be written more simply as:</p>
<pre class="verbatim"><ol><li>    <span class="i">$x</span> += <span class="s">(</span><span class="i">$x</span> % <span class="n">2</span><span class="s">)</span> ? <span class="n">10</span> <span class="co">:</span> <span class="n">2</span><span class="sc">;</span></li></ol></pre><a name="Assignment-Operators"></a><h2>Assignment Operators
      
        
     </h2>
<p><code class="inline"><span class="q">&quot;=&quot;</span></code>
 is the ordinary assignment operator.</p>
<p>Assignment operators work as in C.  That is,</p>
<pre class="verbatim"><ol><li>    <span class="i">$x</span> += <span class="n">2</span><span class="sc">;</span></li></ol></pre><p>is equivalent to</p>
<pre class="verbatim"><ol><li>    <span class="i">$x</span> = <span class="i">$x</span> + <span class="n">2</span><span class="sc">;</span></li></ol></pre><p>although without duplicating any side effects that dereferencing the lvalue
might trigger, such as from <code class="inline"><a class="l_k" href="functions/tie.html">tie()</a></code>.  Other assignment operators work similarly.
The following are recognized:</p>
<pre class="verbatim"><ol><li>    <span class="i">**</span>=    +=    <span class="i">*=</span>    &amp;=    <span class="i">&amp;.</span>=    &lt;&lt;=    &amp;&amp;=</li><li>           -=    <span class="q">/=    |=    |.=    &gt;&gt;=    ||=</span></li><li>           <span class="q">           .=    %=    ^=    ^.=           /</span>/=</li><li>                 <span class="w">x</span>=</li></ol></pre><p>Although these are grouped by family, they all have the precedence
of assignment.  These combined assignment operators can only operate on
scalars, whereas the ordinary assignment operator can assign to arrays,
hashes, lists and even references.  (See <a href="perldata.html#Context">Context</a>
and <a href="perldata.html#List-value-constructors">List value constructors in perldata</a>, and <a href="perlref.html#Assigning-to-References">Assigning to References in perlref</a>.)</p>
<p>Unlike in C, the scalar assignment operator produces a valid lvalue.
Modifying an assignment is equivalent to doing the assignment and
then modifying the variable that was assigned to.  This is useful
for modifying a copy of something, like this:</p>
<pre class="verbatim"><ol><li>    <span class="s">(</span><span class="i">$tmp</span> = <span class="i">$global</span><span class="s">)</span> =~ <span class="q">tr/13579/24680/</span><span class="sc">;</span></li></ol></pre><p>Although as of 5.14, that can be also be accomplished this way:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/use.html">use</a> <span class="v">v5.14</span><span class="sc">;</span></li><li>    <span class="i">$tmp</span> = <span class="s">(</span><span class="i">$global</span> =~  <span class="q">tr/13579/24680/</span><span class="w">r</span><span class="s">)</span><span class="sc">;</span></li></ol></pre><p>Likewise,</p>
<pre class="verbatim"><ol><li>    <span class="s">(</span><span class="i">$x</span> += <span class="n">2</span><span class="s">)</span> *= <span class="n">3</span><span class="sc">;</span></li></ol></pre><p>is equivalent to</p>
<pre class="verbatim"><ol><li>    <span class="i">$x</span> += <span class="n">2</span><span class="sc">;</span></li><li>    <span class="i">$x</span> *= <span class="n">3</span><span class="sc">;</span></li></ol></pre><p>Similarly, a list assignment in list context produces the list of
lvalues assigned to, and a list assignment in scalar context returns
the number of elements produced by the expression on the right hand
side of the assignment.</p>
<p>The three dotted bitwise assignment operators (<code class="inline"><span class="i">&amp;.</span>=</code>
 <code class="inline">|.=</code>
 <code class="inline">^.=</code>
) are new in
Perl 5.22 and experimental.  See <a href="#Bitwise-String-Operators">Bitwise String Operators</a>.</p>
<a name="Comma-Operator"></a><h2>Comma Operator
  </h2>
<p>Binary <code class="inline"><span class="q">&quot;,&quot;</span></code>
 is the comma operator.  In scalar context it evaluates
its left argument, throws that value away, then evaluates its right
argument and returns that value.  This is just like C's comma operator.</p>
<p>In list context, it's just the list argument separator, and inserts
both its arguments into the list.  These arguments are also evaluated
from left to right.</p>
<p>The <code class="inline"><span class="cm">=&gt;</span></code>
 operator (sometimes pronounced "fat comma") is a synonym
for the comma except that it causes a
word on its left to be interpreted as a string if it begins with a letter
or underscore and is composed only of letters, digits and underscores.
This includes operands that might otherwise be interpreted as operators,
constants, single number v-strings or function calls.  If in doubt about
this behavior, the left operand can be quoted explicitly.</p>
<p>Otherwise, the <code class="inline"><span class="cm">=&gt;</span></code>
 operator behaves exactly as the comma operator
or list argument separator, according to context.</p>
<p>For example:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/use.html">use</a> <span class="w">constant</span> <span class="i">FOO</span> <span class="cm">=&gt;</span> <span class="q">&quot;something&quot;</span><span class="sc">;</span></li><li></li><li>    <a class="l_k" href="functions/my.html">my</a> <span class="i">%h</span> = <span class="s">(</span> <span class="i">FOO</span> <span class="cm">=&gt;</span> <span class="n">23</span> <span class="s">)</span><span class="sc">;</span></li></ol></pre><p>is equivalent to:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/my.html">my</a> <span class="i">%h</span> = <span class="s">(</span><span class="q">&quot;FOO&quot;</span><span class="cm">,</span> <span class="n">23</span><span class="s">)</span><span class="sc">;</span></li></ol></pre><p>It is <i>NOT</i>:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/my.html">my</a> <span class="i">%h</span> = <span class="s">(</span><span class="q">&quot;something&quot;</span><span class="cm">,</span> <span class="n">23</span><span class="s">)</span><span class="sc">;</span></li></ol></pre><p>The <code class="inline"><span class="cm">=&gt;</span></code>
 operator is helpful in documenting the correspondence
between keys and values in hashes, and other paired elements in lists.</p>
<pre class="verbatim"><ol><li>    <span class="i">%hash</span> = <span class="s">(</span> <span class="i">$key</span> <span class="cm">=&gt;</span> <span class="i">$value</span> <span class="s">)</span><span class="sc">;</span></li><li>    <span class="i">login</span><span class="s">(</span> <span class="i">$username</span> <span class="cm">=&gt;</span> <span class="i">$password</span> <span class="s">)</span><span class="sc">;</span></li></ol></pre><p>The special quoting behavior ignores precedence, and hence may apply to
<i>part</i> of the left operand:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/print.html">print</a> <a class="l_k" href="functions/time.html">time</a>.<span class="w">shift</span> <span class="cm">=&gt;</span> <span class="q">&quot;bbb&quot;</span><span class="sc">;</span></li></ol></pre><p>That example prints something like <code class="inline"><span class="q">&quot;1314363215shiftbbb&quot;</span></code>
, because the
<code class="inline"><span class="cm">=&gt;</span></code>
 implicitly quotes the <code class="inline"><a class="l_k" href="functions/shift.html">shift</a></code> immediately on its left, ignoring
the fact that <code class="inline"><a class="l_k" href="functions/time.html">time.shift</a></code> is the entire left operand.</p>
<a name="List-Operators-(Rightward)"></a><h2>List Operators (Rightward)
 </h2>
<p>On the right side of a list operator, the comma has very low precedence,
such that it controls all comma-separated expressions found there.
The only operators with lower precedence are the logical operators
<code class="inline"><span class="q">&quot;and&quot;</span></code>
, <code class="inline"><span class="q">&quot;or&quot;</span></code>
, and <code class="inline"><span class="q">&quot;not&quot;</span></code>
, which may be used to evaluate calls to list
operators without the need for parentheses:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/open.html">open</a> <span class="w">HANDLE</span><span class="cm">,</span> <span class="q">&quot;&lt; :encoding(UTF-8)&quot;</span><span class="cm">,</span> <span class="q">&quot;filename&quot;</span></li><li>        <a class="l_k" href="functions/or.html">or</a> <a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;Can&#39;t open: $!\n&quot;</span><span class="sc">;</span></li></ol></pre><p>However, some people find that code harder to read than writing
it with parentheses:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/open.html">open</a><span class="s">(</span><span class="w">HANDLE</span><span class="cm">,</span> <span class="q">&quot;&lt; :encoding(UTF-8)&quot;</span><span class="cm">,</span> <span class="q">&quot;filename&quot;</span><span class="s">)</span></li><li>        <a class="l_k" href="functions/or.html">or</a> <a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;Can&#39;t open: $!\n&quot;</span><span class="sc">;</span></li></ol></pre><p>in which case you might as well just use the more customary <code class="inline"><span class="q">&quot;||&quot;</span></code>
 operator:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/open.html">open</a><span class="s">(</span><span class="w">HANDLE</span><span class="cm">,</span> <span class="q">&quot;&lt; :encoding(UTF-8)&quot;</span><span class="cm">,</span> <span class="q">&quot;filename&quot;</span><span class="s">)</span></li><li>        || <a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;Can&#39;t open: $!\n&quot;</span><span class="sc">;</span></li></ol></pre><p>See also discussion of list operators in <a href="#Terms-and-List-Operators-(Leftward)">Terms and List Operators (Leftward)</a>.</p>
<a name="Logical-Not"></a><h2>Logical Not
 </h2>
<p>Unary <code class="inline"><span class="q">&quot;not&quot;</span></code>
 returns the logical negation of the expression to its right.
It's the equivalent of <code class="inline"><span class="q">&quot;!&quot;</span></code>
 except for the very low precedence.</p>
<a name="Logical-And"></a><h2>Logical And
 </h2>
<p>Binary <code class="inline"><span class="q">&quot;and&quot;</span></code>
 returns the logical conjunction of the two surrounding
expressions.  It's equivalent to <code class="inline">&amp;&amp;</code> except for the very low
precedence.  This means that it short-circuits: the right
expression is evaluated only if the left expression is true.</p>
<a name="Logical-or-and-Exclusive-Or"></a><h2>Logical or and Exclusive Or
 

 </h2>
<p>Binary <code class="inline"><span class="q">&quot;or&quot;</span></code>
 returns the logical disjunction of the two surrounding
expressions.  It's equivalent to <code class="inline">||</code> except for the very low precedence.
This makes it useful for control flow:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/print.html">print</a> <span class="i">FH</span> <span class="i">$data</span>		or <a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;Can&#39;t write to FH: $!&quot;</span><span class="sc">;</span></li></ol></pre><p>This means that it short-circuits: the right expression is evaluated
only if the left expression is false.  Due to its precedence, you must
be careful to avoid using it as replacement for the <code class="inline">||</code> operator.
It usually works out better for flow control than in assignments:</p>
<pre class="verbatim"><ol><li>    <span class="i">$x</span> = <span class="i">$y</span> <a class="l_k" href="functions/or.html">or</a> <span class="i">$z</span><span class="sc">;</span>              <span class="c"># bug: this is wrong</span></li><li>    <span class="s">(</span><span class="i">$x</span> = <span class="i">$y</span><span class="s">)</span> <a class="l_k" href="functions/or.html">or</a> <span class="i">$z</span><span class="sc">;</span>            <span class="c"># really means this</span></li><li>    <span class="i">$x</span> = <span class="i">$y</span> || <span class="i">$z</span><span class="sc">;</span>              <span class="c"># better written this way</span></li></ol></pre><p>However, when it's a list-context assignment and you're trying to use
<code class="inline">||</code> for control flow, you probably need <code class="inline"><span class="q">&quot;or&quot;</span></code>
 so that the assignment
takes higher precedence.</p>
<pre class="verbatim"><ol><li>    <span class="i">@info</span> = <a class="l_k" href="functions/stat.html">stat</a><span class="s">(</span><span class="i">$file</span><span class="s">)</span> || <a class="l_k" href="functions/die.html">die</a><span class="sc">;</span>     <span class="c"># oops, scalar sense of stat!</span></li><li>    <span class="i">@info</span> = <a class="l_k" href="functions/stat.html">stat</a><span class="s">(</span><span class="i">$file</span><span class="s">)</span> or <a class="l_k" href="functions/die.html">die</a><span class="sc">;</span>     <span class="c"># better, now @info gets its due</span></li></ol></pre><p>Then again, you could always use parentheses.</p>
<p>Binary <code class="inline"><span class="q">&quot;xor&quot;</span></code>
 returns the exclusive-OR of the two surrounding expressions.
It cannot short-circuit (of course).</p>
<p>There is no low precedence operator for defined-OR.</p>
<a name="C-Operators-Missing-From-Perl"></a><h2>C Operators Missing From Perl
  
 </h2>
<p>Here is what C has that Perl doesn't:</p>
<ul>
<li><a name="unary-%26"></a><b>unary &amp;</b>
<p>Address-of operator.  (But see the <code class="inline"><span class="q">&quot;\&quot;</span></code>
 operator for taking a reference.)</p>
</li>
<li><a name="unary-*"></a><b>unary *</b>
<p>Dereference-address operator.  (Perl's prefix dereferencing
operators are typed: <code class="inline"><span class="i">$</span></code>
, <code class="inline"><span class="i">@</span></code>
, <code class="inline"><span class="i">%</span></code>
, and <code class="inline"><span class="i">&amp;</span></code>
.)</p>
</li>
<li><a name="(TYPE)"></a><b>(TYPE)</b>
<p>Type-casting operator.</p>
</li>
</ul>
<a name="Quote-and-Quote-like-Operators"></a><h2>Quote and Quote-like Operators
      
          
 </h2>
<p>While we usually think of quotes as literal values, in Perl they
function as operators, providing various kinds of interpolating and
pattern matching capabilities.  Perl provides customary quote characters
for these behaviors, but also provides a way for you to choose your
quote character for any of them.  In the following table, a <code class="inline"><span class="s">{</span><span class="s">}</span></code>
 represents
any pair of delimiters you choose.</p>
<pre class="verbatim"><ol><li>    <span class="w">Customary</span>  <span class="w">Generic</span>        <span class="w">Meaning</span>	     <span class="w">Interpolates</span></li><li>	<span class="q">&#39;&#39;</span>	 <span class="q">q{}</span>	      <span class="w">Literal</span>		  <a class="l_k" href="functions/no.html">no</a></li><li>	<span class="q">&quot;&quot;</span>	<span class="q">qq{}</span>	      <span class="w">Literal</span>		  <span class="w">yes</span></li><li>	<span class="q">``</span>	<span class="q">qx{}</span>	      <span class="w">Command</span>		  <span class="w">yes</span>*</li><li>		<span class="q">qw{}</span>	     <span class="w">Word</span> <span class="w">list</span>		  <a class="l_k" href="functions/no.html">no</a></li><li>	<span class="q">//</span>	 <span class="q">m{}</span>	   <span class="w">Pattern</span> <span class="w">match</span>	  <span class="w">yes</span>*</li><li>		<span class="q">qr{}</span>	      <span class="w">Pattern</span>		  <span class="w">yes</span>*</li><li>		 <span class="q">s{}{}</span>	    <span class="w">Substitution</span>	  <span class="w">yes</span>*</li><li>		<span class="q">tr{}{}</span>	  <span class="w">Transliteration</span>	  <a class="l_k" href="functions/no.html">no</a> <span class="s">(</span><span class="w">but</span> <span class="w">see</span> <span class="w">below</span><span class="s">)</span></li><li>		 <span class="q">y{}{}</span>	  <span class="w">Transliteration</span>	  <a class="l_k" href="functions/no.html">no</a> <span class="s">(</span><span class="w">but</span> <span class="w">see</span> <span class="w">below</span><span class="s">)</span></li><li>        &lt;&lt;<span class="w">EOF</span>                 <span class="w">here</span>-<span class="w">doc</span>            <span class="w">yes</span>*</li><li></li><li>	<span class="i">* unless</span> <span class="w">the</span> <span class="w">delimiter</span> <span class="w">is</span> <span class="q">&#39;&#39;</span>.</li></ol></pre><p>Non-bracketing delimiters use the same character fore and aft, but the four
sorts of ASCII brackets (round, angle, square, curly) all nest, which means
that</p>
<pre class="verbatim"><ol><li>    <span class="q">q{foo{bar}baz}</span></li></ol></pre><p>is the same as</p>
<pre class="verbatim"><ol><li>    <span class="q">&#39;foo{bar}baz&#39;</span></li></ol></pre><p>Note, however, that this does not always work for quoting Perl code:</p>
<pre class="verbatim"><ol><li>    <span class="i">$s</span> = <span class="q">q{ if($x eq &quot;}</span><span class="q">&quot;) ... }; # WRONG</span></li></ol></pre><p>is a syntax error.  The <code class="inline"><a href="Text/Balanced.html">Text::Balanced</a></code> module (standard as of v5.8,
and from CPAN before then) is able to do this properly.</p>
<p>There can (and in some cases, must) be whitespace between the operator
and the quoting
characters, except when <code class="inline"><span class="c">#</span></code>
 is being used as the quoting character.
<code class="inline"><a class="l_k" href="functions/q.html">q#foo#</a></code> is parsed as the string <code class="inline"><span class="w">foo</span></code>
, while <code class="inline">q #foo#</code> is the
operator <code class="inline"><a class="l_k" href="functions/q.html">q</a></code> followed by a comment.  Its argument will be taken
from the next line.  This allows you to write:</p>
<pre class="verbatim"><ol><li>    <span class="q">s {foo}  # Replace foo</span></li><li>      <span class="q">      {bar}</span>  <span class="c"># with bar.</span></li></ol></pre><p>The cases where whitespace must be used are when the quoting character
is a word character (meaning it matches <code class="inline"><span class="q">/\w/</span></code>
):</p>
<pre class="verbatim"><ol><li>    <span class="q">q XfooX</span> <span class="c"># Works: means the string &#39;foo&#39;</span></li><li>    <span class="w">qXfooX</span>  <span class="c"># WRONG!</span></li></ol></pre><p>The following escape sequences are available in constructs that interpolate,
and in transliterations:
           
</p>
<pre class="verbatim"><ol><li>    <span class="w">Sequence</span>     <span class="w">Note</span>  <span class="w">Description</span></li><li>    \<span class="w">t</span>                  <span class="w">tab</span>               <span class="s">(</span><span class="w">HT</span><span class="cm">,</span> <span class="w">TAB</span><span class="s">)</span></li><li>    \<span class="w">n</span>                  <span class="w">newline</span>           <span class="s">(</span><span class="w">NL</span><span class="s">)</span></li><li>    \<span class="w">r</span>                  <a class="l_k" href="functions/return.html">return</a>            <span class="s">(</span><span class="w">CR</span><span class="s">)</span></li><li>    \<span class="w">f</span>                  <span class="w">form</span> <span class="w">feed</span>         <span class="s">(</span><span class="w">FF</span><span class="s">)</span></li><li>    \<span class="w">b</span>                  <span class="w">backspace</span>         <span class="s">(</span><span class="w">BS</span><span class="s">)</span></li><li>    \<span class="w">a</span>                  <a class="l_k" href="functions/alarm.html">alarm</a> <span class="s">(</span><span class="w">bell</span><span class="s">)</span>      <span class="s">(</span><span class="w">BEL</span><span class="s">)</span></li><li>    \<span class="w">e</span>                  <span class="w">escape</span>            <span class="s">(</span><span class="w">ESC</span><span class="s">)</span></li><li>    \<span class="i">x</span><span class="s">{</span><span class="n">263</span><span class="w">A</span><span class="s">}</span>     <span class="s">[</span><span class="n">1</span><span class="cm">,</span><span class="n">8</span><span class="s">]</span>  <a class="l_k" href="functions/hex.html">hex</a> <span class="w">char</span>          <span class="s">(</span><span class="w">example</span><span class="co">:</span> <span class="w">SMILEY</span><span class="s">)</span></li><li>    \<span class="w">x1b</span>         <span class="s">[</span><span class="n">2</span><span class="cm">,</span><span class="n">8</span><span class="s">]</span>  <span class="w">restricted</span> <span class="w">range</span> <a class="l_k" href="functions/hex.html">hex</a> <span class="w">char</span> <span class="s">(</span><span class="w">example</span><span class="co">:</span> <span class="w">ESC</span><span class="s">)</span></li><li>    \<span class="i">N</span><span class="s">{</span><span class="w">name</span><span class="s">}</span>     <span class="s">[</span><span class="n">3</span><span class="s">]</span>    <span class="w">named</span> <span class="w">Unicode</span> <span class="w">character</span> or <span class="w">character</span> <span class="w">sequence</span></li><li>    \<span class="i">N</span><span class="s">{</span><span class="w">U</span>+<span class="n">263</span><span class="w">D</span><span class="s">}</span>   <span class="s">[</span><span class="n">4</span><span class="cm">,</span><span class="n">8</span><span class="s">]</span>  <span class="w">Unicode</span> <span class="w">character</span> <span class="s">(</span><span class="w">example</span><span class="co">:</span> <span class="w">FIRST</span> <span class="w">QUARTER</span> <span class="w">MOON</span><span class="s">)</span></li><li>    \<span class="w">c</span><span class="s">[</span>          <span class="s">[</span><span class="n">5</span><span class="s">]</span>    <span class="w">control</span> <span class="w">char</span>      <span class="s">(</span><span class="w">example</span><span class="co">:</span> <a class="l_k" href="functions/chr.html">chr</a><span class="s">(</span><span class="n">27</span><span class="s">)</span><span class="s">)</span></li><li>    \<span class="i">o</span><span class="s">{</span><span class="n">23072</span><span class="s">}</span>    <span class="s">[</span><span class="n">6</span><span class="cm">,</span><span class="n">8</span><span class="s">]</span>  <span class="w">octal</span> <span class="w">char</span>        <span class="s">(</span><span class="w">example</span><span class="co">:</span> <span class="w">SMILEY</span><span class="s">)</span></li><li>    \<span class="n">033</span>         <span class="s">[</span><span class="n">7</span><span class="cm">,</span><span class="n">8</span><span class="s">]</span>  <span class="w">restricted</span> <span class="w">range</span> <span class="w">octal</span> <span class="w">char</span>  <span class="s">(</span><span class="w">example</span><span class="co">:</span> <span class="w">ESC</span><span class="s">)</span></li></ol></pre><ul>
<li><a name="%5b1%5d"></a><b>[1]</b>
<p>The result is the character specified by the hexadecimal number between
the braces.  See <a href="#%5b8%5d">[8]</a> below for details on which character.</p>
<p>Only hexadecimal digits are valid between the braces.  If an invalid
character is encountered, a warning will be issued and the invalid
character and all subsequent characters (valid or invalid) within the
braces will be discarded.</p>
<p>If there are no valid digits between the braces, the generated character is
the NULL character (<code class="inline">\<span class="i">x</span><span class="s">{</span><span class="n">00</span><span class="s">}</span></code>
).  However, an explicit empty brace (<code class="inline">\<span class="w">x</span><span class="s">{</span><span class="s">}</span></code>
)
will not cause a warning (currently).</p>
</li>
<li><a name="%5b2%5d"></a><b>[2]</b>
<p>The result is the character specified by the hexadecimal number in the range
0x00 to 0xFF.  See <a href="#%5b8%5d">[8]</a> below for details on which character.</p>
<p>Only hexadecimal digits are valid following <code class="inline">\<span class="w">x</span></code>
.  When <code class="inline">\<span class="w">x</span></code>
 is followed
by fewer than two valid digits, any valid digits will be zero-padded.  This
means that <code class="inline">\<span class="w">x7</span></code>
 will be interpreted as <code class="inline">\<span class="w">x07</span></code>
, and a lone <code class="inline"><span class="q">&quot;\x&quot;</span></code>
 will be
interpreted as <code class="inline">\<span class="w">x00</span></code>
.  Except at the end of a string, having fewer than
two valid digits will result in a warning.  Note that although the warning
says the illegal character is ignored, it is only ignored as part of the
escape and will still be used as the subsequent character in the string.
For example:</p>
<pre class="verbatim"><ol><li>  <span class="w">Original</span>    <span class="w">Result</span>    <span class="w">Warns</span>?</li><li>  <span class="q">&quot;\x7&quot;</span>       <span class="q">&quot;\x07&quot;</span>    <a class="l_k" href="functions/no.html">no</a></li><li>  <span class="q">&quot;\x&quot;</span>        <span class="q">&quot;\x00&quot;</span>    <a class="l_k" href="functions/no.html">no</a></li><li>  <span class="q">&quot;\x7q&quot;</span>      <span class="q">&quot;\x07q&quot;</span>   <span class="w">yes</span></li><li>  <span class="q">&quot;\xq&quot;</span>       <span class="q">&quot;\x00q&quot;</span>   <span class="w">yes</span></li></ol></pre></li>
<li><a name="%5b3%5d"></a><b>[3]</b>
<p>The result is the Unicode character or character sequence given by <i>name</i>.
See <a href="charnames.html">charnames</a>.</p>
</li>
<li><a name="%5b4%5d"></a><b>[4]</b>
<p><code class="inline">\N{U+<i>hexadecimal number</i>}</code> means the Unicode character whose Unicode code
point is <i>hexadecimal number</i>.</p>
</li>
<li><a name="%5b5%5d"></a><b>[5]</b>
<p>The character following <code class="inline">\<span class="w">c</span></code>
 is mapped to some other character as shown in the
table:</p>
<pre class="verbatim"><ol><li> <span class="w">Sequence</span>   <span class="w">Value</span></li><li>   \<span class="w">c</span><span class="i">@      chr</span><span class="s">(</span><span class="n">0</span><span class="s">)</span></li><li>   \<span class="w">cA</span>      <a class="l_k" href="functions/chr.html">chr</a><span class="s">(</span><span class="n">1</span><span class="s">)</span></li><li>   \<span class="w">ca</span>      <a class="l_k" href="functions/chr.html">chr</a><span class="s">(</span><span class="n">1</span><span class="s">)</span></li><li>   \<span class="w">cB</span>      <a class="l_k" href="functions/chr.html">chr</a><span class="s">(</span><span class="n">2</span><span class="s">)</span></li><li>   \<span class="w">cb</span>      <a class="l_k" href="functions/chr.html">chr</a><span class="s">(</span><span class="n">2</span><span class="s">)</span></li><li>   ...</li><li>   \<span class="w">cZ</span>      <a class="l_k" href="functions/chr.html">chr</a><span class="s">(</span><span class="n">26</span><span class="s">)</span></li><li>   \<span class="w">cz</span>      <a class="l_k" href="functions/chr.html">chr</a><span class="s">(</span><span class="n">26</span><span class="s">)</span></li><li>   \<span class="w">c</span><span class="s">[</span>      <a class="l_k" href="functions/chr.html">chr</a><span class="s">(</span><span class="n">27</span><span class="s">)</span></li><li>                     <span class="c"># See below for chr(28)</span></li><li>   \<span class="w">c</span><span class="s">]</span>      <a class="l_k" href="functions/chr.html">chr</a><span class="s">(</span><span class="n">29</span><span class="s">)</span></li><li>   \<span class="w">c</span>^      <a class="l_k" href="functions/chr.html">chr</a><span class="s">(</span><span class="n">30</span><span class="s">)</span></li><li>   \<span class="w">c_</span>      <a class="l_k" href="functions/chr.html">chr</a><span class="s">(</span><span class="n">31</span><span class="s">)</span></li><li>   \<span class="w">c</span>?      <a class="l_k" href="functions/chr.html">chr</a><span class="s">(</span><span class="n">127</span><span class="s">)</span> <span class="c"># (on ASCII platforms; see below for link to</span></li><li>                     <span class="c">#  EBCDIC discussion)</span></li></ol></pre><p>In other words, it's the character whose code point has had 64 xor'd with
its uppercase.  <code class="inline">\<span class="w">c</span>?</code>
 is DELETE on ASCII platforms because
<code class="inline"><a class="l_k" href="functions/ord.html">ord</a><span class="s">(</span><span class="q">&quot;?&quot;</span><span class="s">)</span> ^ <span class="n">64</span></code>
 is 127, and
<code class="inline">\<span class="w">c</span><span class="i">@</span></code>
 is NULL because the ord of <code class="inline"><span class="q">&quot;@&quot;</span></code>
 is 64, so xor'ing 64 itself produces 0.</p>
<p>Also, <code class="inline">\c\<i>X</i></code> yields <code class="inline"> chr(28) . "<i>X</i>"</code> for any <i>X</i>, but cannot come at the
end of a string, because the backslash would be parsed as escaping the end
quote.</p>
<p>On ASCII platforms, the resulting characters from the list above are the
complete set of ASCII controls.  This isn't the case on EBCDIC platforms; see
<a href="perlebcdic.html#OPERATOR-DIFFERENCES">OPERATOR DIFFERENCES in perlebcdic</a> for a full discussion of the
differences between these for ASCII versus EBCDIC platforms.</p>
<p>Use of any other character following the <code class="inline"><span class="q">&quot;c&quot;</span></code>
 besides those listed above is
discouraged, and as of Perl v5.20, the only characters actually allowed
are the printable ASCII ones, minus the left brace <code class="inline"><span class="q">&quot;{&quot;</span></code>
.  What happens
for any of the allowed other characters is that the value is derived by
xor'ing with the seventh bit, which is 64, and a warning raised if
enabled.  Using the non-allowed characters generates a fatal error.</p>
<p>To get platform independent controls, you can use <code class="inline">\<span class="i">N</span><span class="s">{</span>...<span class="s">}</span></code>
.</p>
</li>
<li><a name="%5b6%5d"></a><b>[6]</b>
<p>The result is the character specified by the octal number between the braces.
See <a href="#%5b8%5d">[8]</a> below for details on which character.</p>
<p>If a character that isn't an octal digit is encountered, a warning is raised,
and the value is based on the octal digits before it, discarding it and all
following characters up to the closing brace.  It is a fatal error if there are
no octal digits at all.</p>
</li>
<li><a name="%5b7%5d"></a><b>[7]</b>
<p>The result is the character specified by the three-digit octal number in the
range 000 to 777 (but best to not use above 077, see next paragraph).  See
<a href="#%5b8%5d">[8]</a> below for details on which character.</p>
<p>Some contexts allow 2 or even 1 digit, but any usage without exactly
three digits, the first being a zero, may give unintended results.  (For
example, in a regular expression it may be confused with a backreference;
see <a href="perlrebackslash.html#Octal-escapes">Octal escapes in perlrebackslash</a>.)  Starting in Perl 5.14, you may
use <code class="inline">\<span class="w">o</span><span class="s">{</span><span class="s">}</span></code>
 instead, which avoids all these problems.  Otherwise, it is best to
use this construct only for ordinals <code class="inline">\<span class="n">077</span></code>
 and below, remembering to pad to
the left with zeros to make three digits.  For larger ordinals, either use
<code class="inline">\<span class="w">o</span><span class="s">{</span><span class="s">}</span></code>
, or convert to something else, such as to hex and use <code class="inline">\<span class="i">N</span><span class="s">{</span><span class="w">U</span>+<span class="s">}</span></code>

(which is portable between platforms with different character sets) or
<code class="inline">\<span class="w">x</span><span class="s">{</span><span class="s">}</span></code>
 instead.</p>
</li>
<li><a name="%5b8%5d"></a><b>[8]</b>
<p>Several constructs above specify a character by a number.  That number
gives the character's position in the character set encoding (indexed from 0).
This is called synonymously its ordinal, code position, or code point.  Perl
works on platforms that have a native encoding currently of either ASCII/Latin1
or EBCDIC, each of which allow specification of 256 characters.  In general, if
the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's
native encoding.  If the number is 256 (0x100, 0400) or above, Perl interprets
it as a Unicode code point and the result is the corresponding Unicode
character.  For example <code class="inline">\<span class="i">x</span><span class="s">{</span><span class="n">50</span><span class="s">}</span></code>
 and <code class="inline">\<span class="i">o</span><span class="s">{</span><span class="n">120</span><span class="s">}</span></code>
 both are the number 80 in
decimal, which is less than 256, so the number is interpreted in the native
character set encoding.  In ASCII the character in the 80th position (indexed
from 0) is the letter <code class="inline"><span class="q">&quot;P&quot;</span></code>
, and in EBCDIC it is the ampersand symbol <code class="inline"><span class="q">&quot;&amp;&quot;</span></code>
.
<code class="inline">\<span class="i">x</span><span class="s">{</span><span class="n">100</span><span class="s">}</span></code>
 and <code class="inline">\<span class="i">o</span><span class="s">{</span><span class="n">400</span><span class="s">}</span></code>
 are both 256 in decimal, so the number is interpreted
as a Unicode code point no matter what the native encoding is.  The name of the
character in the 256th position (indexed by 0) in Unicode is
<code class="inline"><span class="w">LATIN</span> <span class="w">CAPITAL</span> <span class="w">LETTER</span> <span class="w">A</span> <span class="w">WITH</span> <span class="w">MACRON</span></code>
.</p>
<p>An exception to the above rule is that <code class="inline">\N{U+<i>hex number</i>}</code> is
always interpreted as a Unicode code point, so that <code class="inline">\<span class="i">N</span><span class="s">{</span><span class="w">U</span>+<span class="n">0050</span><span class="s">}</span></code>
 is <code class="inline"><span class="q">&quot;P&quot;</span></code>
 even
on EBCDIC platforms.</p>
</li>
</ul>
<p><b>NOTE</b>: Unlike C and other languages, Perl has no <code class="inline">\<span class="w">v</span></code>
 escape sequence for
the vertical tab (VT, which is 11 in both ASCII and EBCDIC), but you may
use <code class="inline">\<span class="i">N</span><span class="s">{</span><span class="w">VT</span><span class="s">}</span></code>
, <code class="inline">\<span class="w">ck</span></code>
, <code class="inline">\<span class="i">N</span><span class="s">{</span><span class="w">U</span>+<span class="n">0</span><span class="w">b</span><span class="s">}</span></code>
, or <code class="inline">\<span class="w">x0b</span></code>
.  (<code class="inline">\<span class="w">v</span></code>

does have meaning in regular expression patterns in Perl, see <a href="perlre.html">perlre</a>.)</p>
<p>The following escape sequences are available in constructs that interpolate,
but not in transliterations.
      </p>
<pre class="verbatim"><ol><li>    \<span class="w">l</span>		<span class="w">lowercase</span> <a class="l_k" href="functions/next.html">next</a> <span class="j">character</span> <span class="w">only</span></li><li>    \<span class="w">u</span>		<span class="w">titlecase</span> <span class="s">(</span><a class="l_k" href="functions/not.html">not</a> <span class="w">uppercase</span>!<span class="s">)</span> <a class="l_k" href="functions/next.html">next</a> <span class="j">character</span> <span class="w">only</span></li><li>    \<span class="w">L</span>		<span class="w">lowercase</span> <span class="w">all</span> <span class="w">characters</span> <span class="w">till</span> \<span class="w">E</span> <a class="l_k" href="functions/or.html">or</a> <span class="w">end</span> <span class="w">of</span> <span class="w">string</span></li><li>    \<span class="w">U</span>		<span class="w">uppercase</span> <span class="w">all</span> <span class="w">characters</span> <span class="w">till</span> \<span class="w">E</span> <a class="l_k" href="functions/or.html">or</a> <span class="w">end</span> <span class="w">of</span> <span class="w">string</span></li><li>    \<span class="w">F</span>		<span class="w">foldcase</span> <span class="w">all</span> <span class="w">characters</span> <span class="w">till</span> \<span class="w">E</span> <a class="l_k" href="functions/or.html">or</a> <span class="w">end</span> <span class="w">of</span> <span class="w">string</span></li><li>    \<span class="w">Q</span>          <span class="w">quote</span> <span class="s">(</span><span class="w">disable</span><span class="s">)</span> <span class="w">pattern</span> <span class="w">metacharacters</span> <span class="w">till</span> \<span class="w">E</span> <a class="l_k" href="functions/or.html">or</a></li><li>                <span class="w">end</span> <span class="w">of</span> <span class="w">string</span></li><li>    \<span class="w">E</span>		<span class="w">end</span> <span class="w">either</span> case <span class="w">modification</span> <a class="l_k" href="functions/or.html">or</a> <span class="w">quoted</span> <span class="i">section</span></li><li>		<span class="s">(</span><span class="w">whichever</span> <span class="w">was</span> <a class="l_k" href="functions/last.html">last</a> <span class="j">seen</span><span class="s">)</span></li></ol></pre><p>See <a href="functions/quotemeta.html">quotemeta</a> for the exact definition of characters that
are quoted by <code class="inline">\<span class="w">Q</span></code>
.</p>
<p><code class="inline">\<span class="w">L</span></code>
, <code class="inline">\<span class="w">U</span></code>
, <code class="inline">\<span class="w">F</span></code>
, and <code class="inline">\<span class="w">Q</span></code>
 can stack, in which case you need one
<code class="inline">\<span class="w">E</span></code>
 for each.  For example:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/say.html">say</a><span class="q">&quot;This \Qquoting \ubusiness \Uhere isn&#39;t quite\E done yet,\E is it?&quot;</span><span class="sc">;</span></li><li> <span class="w">This</span> <span class="w">quoting</span>\ <span class="w">Business</span>\ <span class="w">HERE</span>\ <span class="w">ISN</span>\<span class="q">&#39;T\ QUITE\ done\ yet\, is it?</span></li></ol></pre><p>If a <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">locale</span></code>
 form that includes <code class="inline"><span class="w">LC_CTYPE</span></code>
 is in effect (see
<a href="perllocale.html">perllocale</a>), the case map used by <code class="inline">\<span class="w">l</span></code>
, <code class="inline">\<span class="w">L</span></code>
, <code class="inline">\<span class="w">u</span></code>
, and <code class="inline">\<span class="w">U</span></code>
 is
taken from the current locale.  If Unicode (for example, <code class="inline">\<span class="w">N</span><span class="s">{</span><span class="s">}</span></code>
 or code
points of 0x100 or beyond) is being used, the case map used by <code class="inline">\<span class="w">l</span></code>
,
<code class="inline">\<span class="w">L</span></code>
, <code class="inline">\<span class="w">u</span></code>
, and <code class="inline">\<span class="w">U</span></code>
 is as defined by Unicode.  That means that
case-mapping a single character can sometimes produce a sequence of
several characters.
Under <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">locale</span></code>
, <code class="inline">\<span class="w">F</span></code>
 produces the same results as <code class="inline">\<span class="w">L</span></code>

for all locales but a UTF-8 one, where it instead uses the Unicode
definition.</p>
<p>All systems use the virtual <code class="inline"><span class="q">&quot;\n&quot;</span></code>
 to represent a line terminator,
called a "newline".  There is no such thing as an unvarying, physical
newline character.  It is only an illusion that the operating system,
device drivers, C libraries, and Perl all conspire to preserve.  Not all
systems read <code class="inline"><span class="q">&quot;\r&quot;</span></code>
 as ASCII CR and <code class="inline"><span class="q">&quot;\n&quot;</span></code>
 as ASCII LF.  For example,
on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed,
and on systems without a line terminator,
printing <code class="inline"><span class="q">&quot;\n&quot;</span></code>
 might emit no actual data.  In general, use <code class="inline"><span class="q">&quot;\n&quot;</span></code>
 when
you mean a "newline" for your system, but use the literal ASCII when you
need an exact character.  For example, most networking protocols expect
and prefer a CR+LF (<code class="inline"><span class="q">&quot;\015\012&quot;</span></code>
 or <code class="inline"><span class="q">&quot;\cM\cJ&quot;</span></code>
) for line terminators,
and although they often accept just <code class="inline"><span class="q">&quot;\012&quot;</span></code>
, they seldom tolerate just
<code class="inline"><span class="q">&quot;\015&quot;</span></code>
.  If you get in the habit of using <code class="inline"><span class="q">&quot;\n&quot;</span></code>
 for networking,
you may be burned some day.
   
  </p>
<p>For constructs that do interpolate, variables beginning with "<code class="inline"><span class="i">$</span></code>
"
or "<code class="inline"><span class="i">@</span></code>
" are interpolated.  Subscripted variables such as <code class="inline"><span class="i">$a</span>[<span class="n">3</span>]</code>
 or
<code class="inline"><span class="i">$href</span>-&gt;{<span class="w">key</span>}[<span class="n">0</span>]</code>
 are also interpolated, as are array and hash slices.
But method calls such as <code class="inline"><span class="i">$obj</span><span class="i">-&gt;meth</span></code>
 are not.</p>
<p>Interpolating an array or slice interpolates the elements in order,
separated by the value of <code class="inline"><span class="i">$&quot;</span></code>
, so is equivalent to interpolating
<code class="inline"><a class="l_k" href="functions/join.html">join</a> <span class="i">$&quot;</span><span class="cm">,</span> <span class="i">@array</span></code>
.  "Punctuation" arrays such as <code class="inline"><span class="i">@*</span></code>
 are usually
interpolated only if the name is enclosed in braces <code class="inline">@{*}</code>, but the
arrays <code class="inline"><span class="i">@_</span></code>
, <code class="inline"><span class="i">@+</span></code>
, and <code class="inline"><span class="i">@-</span></code>
 are interpolated even without braces.</p>
<p>For double-quoted strings, the quoting from <code class="inline">\<span class="w">Q</span></code>
 is applied after
interpolation and escapes are processed.</p>
<pre class="verbatim"><ol><li>    <span class="q">&quot;abc\Qfoo\tbar$s\Exyz&quot;</span></li></ol></pre><p>is equivalent to</p>
<pre class="verbatim"><ol><li>    <span class="q">&quot;abc&quot;</span> . <a class="l_k" href="functions/quotemeta.html">quotemeta</a><span class="s">(</span><span class="q">&quot;foo\tbar$s&quot;</span><span class="s">)</span> . <span class="q">&quot;xyz&quot;</span></li></ol></pre><p>For the pattern of regex operators (<code class="inline"><a class="l_k" href="functions/qr.html">qr//</a></code>, <code class="inline"><a class="l_k" href="functions/m.html">m//</a></code> and <code class="inline"><a class="l_k" href="functions/s.html">s///</a></code>),
the quoting from <code class="inline">\<span class="w">Q</span></code>
 is applied after interpolation is processed,
but before escapes are processed.  This allows the pattern to match
literally (except for <code class="inline"><span class="i">$</span></code>
 and <code class="inline"><span class="i">@</span></code>
).  For example, the following matches:</p>
<pre class="verbatim"><ol><li>    <span class="q">&#39;\s\t&#39;</span> =~ <span class="q">/\Q\s\t/</span></li></ol></pre><p>Because <code class="inline"><span class="i">$</span></code>
 or <code class="inline"><span class="i">@</span></code>
 trigger interpolation, you'll need to use something
like <code class="inline"><span class="q">/\Quser\E\@\Qhost/</span></code>
 to match them literally.</p>
<p>Patterns are subject to an additional level of interpretation as a
regular expression.  This is done as a second pass, after variables are
interpolated, so that regular expressions may be incorporated into the
pattern from the variables.  If this is not what you want, use <code class="inline">\<span class="w">Q</span></code>
 to
interpolate a variable literally.</p>
<p>Apart from the behavior described above, Perl does not expand
multiple levels of interpolation.  In particular, contrary to the
expectations of shell programmers, back-quotes do <i>NOT</i> interpolate
within double quotes, nor do single quotes impede evaluation of
variables when used within double quotes.</p>
<a name="Regexp-Quote-Like-Operators"></a><h2>Regexp Quote-Like Operators
</h2>
<p>Here are the quote-like operators that apply to pattern
matching and related activities.</p>
<ul>
<li><a name="qr%2f_STRING_%2fmsixpodualn"></a><b><code class="inline">qr/<i>STRING</i>/msixpodualn</code>
      </b>
<p>This operator quotes (and possibly compiles) its <i>STRING</i> as a regular
expression.  <i>STRING</i> is interpolated the same way as <i>PATTERN</i>
in <code class="inline">m/<i>PATTERN</i>/</code>.  If <code class="inline"><span class="q">&quot;&#39;&quot;</span></code>
 is used as the delimiter, no variable
interpolation is done.  Returns a Perl value which may be used instead of the
corresponding <code class="inline">/<i>STRING</i>/msixpodualn</code> expression.  The returned value is a
normalized version of the original pattern.  It magically differs from
a string containing the same characters: <code class="inline"><a class="l_k" href="functions/ref.html">ref(qr/x/)</a></code> returns "Regexp";
however, dereferencing it is not well defined (you currently get the
normalized version of the original pattern, but this may change).</p>
<p>For example,</p>
<pre class="verbatim"><ol><li>    <span class="i">$rex</span> = <span class="q">qr/my.STRING/is</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/print.html">print</a> <span class="i">$rex</span><span class="sc">;</span>                 <span class="c"># prints (?si-xm:my.STRING)</span></li><li>    <span class="q">s/$rex/foo/</span><span class="sc">;</span></li></ol></pre><p>is equivalent to</p>
<pre class="verbatim"><ol><li>    <span class="q">s/my.STRING/foo/is</span><span class="sc">;</span></li></ol></pre><p>The result may be used as a subpattern in a match:</p>
<pre class="verbatim"><ol><li>    <span class="i">$re</span> = <span class="q">qr/$pattern/</span><span class="sc">;</span></li><li>    <span class="i">$string</span> =~ <span class="q">/foo${re}bar/</span><span class="sc">;</span>	<span class="c"># can be interpolated in other</span></li><li>                                <span class="c"># patterns</span></li><li>    <span class="i">$string</span> =~ <span class="i">$re</span><span class="sc">;</span>		<span class="c"># or used standalone</span></li><li>    <span class="i">$string</span> =~ <span class="q">/$re/</span><span class="sc">;</span>		<span class="c"># or this way</span></li></ol></pre><p>Since Perl may compile the pattern at the moment of execution of the <code class="inline"><a class="l_k" href="functions/qr.html">qr()</a></code>
operator, using <code class="inline"><a class="l_k" href="functions/qr.html">qr()</a></code> may have speed advantages in some situations,
notably if the result of <code class="inline"><a class="l_k" href="functions/qr.html">qr()</a></code> is used standalone:</p>
<pre class="verbatim"><ol><li><a name="match"></a>    sub <span class="m">match</span> <span class="s">{</span></li><li>	<a class="l_k" href="functions/my.html">my</a> <span class="i">$patterns</span> = <a class="l_k" href="functions/shift.html">shift</a><span class="sc">;</span></li><li>	<a class="l_k" href="functions/my.html">my</a> <span class="i">@compiled</span> = <a class="l_k" href="functions/map.html">map</a> <span class="q">qr/$_/i</span><span class="cm">,</span> <span class="i">@$patterns</span><span class="sc">;</span></li><li>	<a class="l_k" href="functions/grep.html">grep</a> <span class="s">{</span></li><li>	    <a class="l_k" href="functions/my.html">my</a> <span class="i">$success</span> = <span class="n">0</span><span class="sc">;</span></li><li>	    foreach <a class="l_k" href="functions/my.html">my</a> <span class="i">$pat</span> <span class="s">(</span><span class="i">@compiled</span><span class="s">)</span> <span class="s">{</span></li><li>		<span class="i">$success</span> = <span class="n">1</span><span class="cm">,</span> <a class="l_k" href="functions/last.html">last</a> if <span class="q">/$pat/</span><span class="sc">;</span></li><li>	    <span class="s">}</span></li><li>	    <span class="i">$success</span><span class="sc">;</span></li><li>	<span class="s">}</span> <span class="i">@_</span><span class="sc">;</span></li><li>    <span class="s">}</span></li></ol></pre><p>Precompilation of the pattern into an internal representation at
the moment of <code class="inline"><a class="l_k" href="functions/qr.html">qr()</a></code> avoids the need to recompile the pattern every
time a match <code class="inline"><span class="q">/$pat/</span></code>
 is attempted.  (Perl has many other internal
optimizations, but none would be triggered in the above example if
we did not use <code class="inline"><a class="l_k" href="functions/qr.html">qr()</a></code> operator.)</p>
<p>Options (specified by the following modifiers) are:</p>
<pre class="verbatim"><ol><li>    <span class="q">m	Treat string as multiple lines.</span></li><li>    <span class="q">    s	Treat</span> <span class="w">string</span> <span class="w">as</span> <span class="w">single</span> <span class="w">line</span>. <span class="s">(</span><span class="w">Make</span> . <span class="w">match</span> <span class="w">a</span> <span class="w">newline</span><span class="s">)</span></li><li>    <span class="w">i</span>	<span class="w">Do</span> case-<span class="w">insensitive</span> <span class="w">pattern</span> <span class="w">matching</span>.</li><li>    <span class="w">x</span>   <span class="w">Use</span> <span class="w">extended</span> <span class="w">regular</span> <span class="w">expressions</span><span class="sc">;</span> <span class="w">specifying</span> <span class="w">two</span></li><li>        <span class="w">x&#39;s</span> <span class="w">means</span> \<span class="w">t</span> <a class="l_k" href="functions/and.html">and</a> <span class="w">the</span> <span class="w">SPACE</span> <span class="w">character</span> <span class="w">are</span> <span class="w">ignored</span> <span class="w">within</span></li><li>        <span class="w">square</span>-<span class="w">bracketed</span> <span class="w">character</span> <span class="w">classes</span></li><li>    <span class="w">p</span>	<span class="w">When</span> <span class="w">matching</span> <span class="w">preserve</span> <span class="w">a</span> <span class="w">copy</span> <span class="w">of</span> <span class="w">the</span> <span class="w">matched</span> <span class="w">string</span> <span class="w">so</span></li><li>        <span class="w">that</span> <span class="i">$</span>{<span class="w">^PREMATCH</span>}<span class="cm">,</span> <span class="i">$</span>{<span class="w">^MATCH</span>}<span class="cm">,</span> <span class="i">$</span>{<span class="w">^POSTMATCH</span>} <span class="w">will</span> <span class="w">be</span></li><li>        <a class="l_k" href="functions/defined.html">defined</a> <span class="s">(</span><span class="w">ignored</span> <span class="w">starting</span> <span class="w">in</span> <span class="v">v5.20</span><span class="s">)</span> <span class="w">as</span> <span class="w">these</span> <span class="w">are</span> <span class="w">always</span></li><li>        <a class="l_k" href="functions/defined.html">defined</a> <span class="w">starting</span> <span class="w">in</span> <span class="w">that</span> <span class="w">release</span></li><li>    <span class="w">o</span>	<span class="w">Compile</span> <span class="w">pattern</span> <span class="w">only</span> <span class="w">once</span>.</li><li>    <span class="w">a</span>   <span class="w">ASCII</span>-<span class="w">restrict</span><span class="co">:</span> <span class="w">Use</span> <span class="w">ASCII</span> <a class="l_k" href="functions/for.html">for</a> \<span class="w">d</span><span class="cm">,</span> \<span class="q">s, \w and [[:posix:]]</span></li><li>        <span class="q">        character classes; specifying two a&#39;s adds the further</span></li><li>        <span class="q">        restriction that no ASCII character will match a</span></li><li>        <span class="q">        non-ASCII one under /i.</span></li><li>    <span class="q">    l   Use the current run-time locale&#39;s rules.</span></li><li>    <span class="q">    u   Use Unicode rules.</span></li><li>    <span class="q">    d   Use Unicode or native charset, as in 5.12 and earlier.</span></li><li>    <span class="q">    n   Non-capture mode. Don&#39;t let () fill in $1,</span> <span class="i">$2</span><span class="cm">,</span> <span class="w">etc</span>...</li></ol></pre><p>If a precompiled pattern is embedded in a larger pattern then the effect
of <code class="inline"><span class="q">&quot;msixpluadn&quot;</span></code>
 will be propagated appropriately.  The effect that the
<code class="inline">/o</code> modifier has is not propagated, being restricted to those patterns
explicitly using it.</p>
<p>The last four modifiers listed above, added in Perl 5.14,
control the character set rules, but <code class="inline"><span class="q">/a</span></code>
 is the only one you are likely
to want to specify explicitly; the other three are selected
automatically by various pragmas.</p>
<p>See <a href="perlre.html">perlre</a> for additional information on valid syntax for <i>STRING</i>, and
for a detailed look at the semantics of regular expressions.  In
particular, all modifiers except the largely obsolete <code class="inline">/o</code> are further
explained in <a href="perlre.html#Modifiers">Modifiers in perlre</a>.  <code class="inline">/o</code> is described in the next section.</p>
</li>
<li><a name="m%2f_PATTERN_%2fmsixpodualngc"></a><b><code class="inline">m/<i>PATTERN</i>/msixpodualngc</code>
 
   
       </b>
</li>
<li><a name="%2f_PATTERN_%2fmsixpodualngc"></a><b><code class="inline">/<i>PATTERN</i>/msixpodualngc</code></b>
<p>Searches a string for a pattern match, and in scalar context returns
true if it succeeds, false if it fails.  If no string is specified
via the <code class="inline">=~</code>
 or <code class="inline">!~</code>
 operator, the <code class="inline"><span class="i">$_</span></code>
 string is searched.  (The
string specified with <code class="inline">=~</code>
 need not be an lvalue--it may be the
result of an expression evaluation, but remember the <code class="inline">=~</code>
 binds
rather tightly.)  See also <a href="perlre.html">perlre</a>.</p>
<p>Options are as described in <code class="inline"><a class="l_k" href="functions/qr.html">qr//</a></code> above; in addition, the following match
process modifiers are available:</p>
<pre class="verbatim"><ol><li> <span class="w">g</span>  <span class="w">Match</span> <span class="w">globally</span><span class="cm">,</span> <span class="w">i</span>.<span class="w">e</span>.<span class="cm">,</span> <span class="w">find</span> <span class="w">all</span> <span class="w">occurrences</span>.</li><li> <span class="w">c</span>  <span class="w">Do</span> <a class="l_k" href="functions/not.html">not</a> <a class="l_k" href="functions/reset.html">reset</a> <span class="w">search</span> <span class="w">position</span> <span class="w">on</span> <span class="w">a</span> <span class="w">failed</span> <span class="w">match</span> <a class="l_k" href="functions/when.html">when</a> <span class="q">/g is</span></li><li>    <span class="q">    in effect.</span></li></ol></pre><p>If <code class="inline"><span class="q">&quot;/&quot;</span></code>
 is the delimiter then the initial <code class="inline"><a class="l_k" href="functions/m.html">m</a></code> is optional.  With the <code class="inline"><a class="l_k" href="functions/m.html">m</a></code>
you can use any pair of non-whitespace (ASCII) characters
as delimiters.  This is particularly useful for matching path names
that contain <code class="inline"><span class="q">&quot;/&quot;</span></code>
, to avoid LTS (leaning toothpick syndrome).  If <code class="inline"><span class="q">&quot;?&quot;</span></code>
 is
the delimiter, then a match-only-once rule applies,
described in <code class="inline">m?<i>PATTERN</i>?</code> below.  If <code class="inline"><span class="q">&quot;&#39;&quot;</span></code>
 (single quote) is the delimiter,
no variable interpolation is performed on the <i>PATTERN</i>.
When using a delimiter character valid in an identifier, whitespace is required
after the <code class="inline"><a class="l_k" href="functions/m.html">m</a></code>.</p>
<p><i>PATTERN</i> may contain variables, which will be interpolated
every time the pattern search is evaluated, except
for when the delimiter is a single quote.  (Note that <code class="inline"><span class="i">$(</span></code>
, <code class="inline"><span class="i">$)</span></code>
, and
<code class="inline"><span class="i">$|</span></code>
 are not interpolated because they look like end-of-string tests.)
Perl will not recompile the pattern unless an interpolated
variable that it contains changes.  You can force Perl to skip the
test and never recompile by adding a <code class="inline">/o</code> (which stands for "once")
after the trailing delimiter.
Once upon a time, Perl would recompile regular expressions
unnecessarily, and this modifier was useful to tell it not to do so, in the
interests of speed.  But now, the only reasons to use <code class="inline">/o</code> are one of:</p>
<dl>
<dt>1</dt><dd>
<p>The variables are thousands of characters long and you know that they
don't change, and you need to wring out the last little bit of speed by
having Perl skip testing for that.  (There is a maintenance penalty for
doing this, as mentioning <code class="inline">/o</code> constitutes a promise that you won't
change the variables in the pattern.  If you do change them, Perl won't
even notice.)</p>
</dd>
<dt>2</dt><dd>
<p>you want the pattern to use the initial values of the variables
regardless of whether they change or not.  (But there are saner ways
of accomplishing this than using <code class="inline">/o</code>.)</p>
</dd>
<dt>3</dt><dd>
<p>If the pattern contains embedded code, such as</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/use.html">use</a> <span class="w">re</span> <span class="q">&#39;eval&#39;</span><span class="sc">;</span></li><li>    <span class="i">$code</span> = <span class="q">&#39;foo(?{ $x })&#39;</span><span class="sc">;</span></li><li>    <span class="q">/$code/</span></li></ol></pre><p>then perl will recompile each time, even though the pattern string hasn't
changed, to ensure that the current value of <code class="inline"><span class="i">$x</span></code>
 is seen each time.
Use <code class="inline">/o</code> if you want to avoid this.</p>
</dd>
</dl>
<p>The bottom line is that using <code class="inline">/o</code> is almost never a good idea.</p>
</li>
<li><a name="The-empty-pattern-%2f%2f"></a><b>The empty pattern <code class="inline"><span class="q">//</span></code>
</b>
<p>If the <i>PATTERN</i> evaluates to the empty string, the last
<i>successfully</i> matched regular expression is used instead.  In this
case, only the <code class="inline"><span class="w">g</span></code>
 and <code class="inline"><span class="w">c</span></code>
 flags on the empty pattern are honored;
the other flags are taken from the original pattern.  If no match has
previously succeeded, this will (silently) act instead as a genuine
empty pattern (which will always match).</p>
<p>Note that it's possible to confuse Perl into thinking <code class="inline"><span class="q">//</span></code>
 (the empty
regex) is really <code class="inline"><span class="q">//</span></code>
 (the defined-or operator).  Perl is usually pretty
good about this, but some pathological cases might trigger this, such as
<code class="inline"><span class="i">$x</span>//<span class="q">/</span></code>
 (is that <code class="inline"><span class="s">(</span><span class="i">$x</span><span class="s">)</span> / <span class="s">(</span><span class="q">//</span><span class="s">)</span></code>
 or <code class="inline"><span class="i">$x</span> // <span class="q">/</span></code>
?) and <code class="inline"><a class="l_k" href="functions/print.html">print</a> <span class="i">$fh</span> <span class="q">//</span></code>

(<code class="inline">print $fh(//</code> or <code class="inline">print($fh //</code>?).  In all of these examples, Perl
will assume you meant defined-or.  If you meant the empty regex, just
use parentheses or spaces to disambiguate, or even prefix the empty
regex with an <code class="inline"><a class="l_k" href="functions/m.html">m</a></code> (so <code class="inline"><span class="q">//</span></code>
 becomes <code class="inline"><a class="l_k" href="functions/m.html">m//</a></code>).</p>
</li>
<li><a name="Matching-in-list-context"></a><b>Matching in list context</b>
<p>If the <code class="inline">/g</code> option is not used, <code class="inline"><a class="l_k" href="functions/m.html">m//</a></code> in list context returns a
list consisting of the subexpressions matched by the parentheses in the
pattern, that is, (<code class="inline"><span class="i">$1</span></code>
, <code class="inline"><span class="i">$2</span></code>
, <code class="inline"><span class="i">$3</span></code>
...)  (Note that here <code class="inline"><span class="i">$1</span></code>
 etc. are
also set).  When there are no parentheses in the pattern, the return
value is the list <code class="inline"><span class="s">(</span><span class="n">1</span><span class="s">)</span></code>
 for success.
With or without parentheses, an empty list is returned upon failure.</p>
<p>Examples:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/open.html">open</a><span class="s">(</span><span class="w">TTY</span><span class="cm">,</span> <span class="q">&quot;+&lt;/dev/tty&quot;</span><span class="s">)</span></li><li>    || <a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;can&#39;t access /dev/tty: $!&quot;</span><span class="sc">;</span></li><li></li><li> <span class="q">&lt;TTY&gt;</span> =~ <span class="q">/^y/i</span> &amp;&amp; <span class="i">foo</span><span class="s">(</span><span class="s">)</span><span class="sc">;</span>	<span class="c"># do foo if desired</span></li><li></li><li> <a class="l_k" href="functions/if.html">if</a> <span class="s">(</span><span class="q">/Version: *([0-9.]*)/</span><span class="s">)</span> <span class="s">{</span> <span class="i">$version</span> = <span class="i">$1</span><span class="sc">;</span> <span class="s">}</span></li><li></li><li> <a class="l_k" href="functions/next.html">next</a> <a class="l_k" href="functions/if.html">if</a> <span class="q">m#^/usr/spool/uucp#</span><span class="sc">;</span></li><li></li><li> <span class="c"># poor man&#39;s grep</span></li><li> <span class="i">$arg</span> = <a class="l_k" href="functions/shift.html">shift</a><span class="sc">;</span></li><li> <a class="l_k" href="functions/while.html">while</a> <span class="s">(</span>&lt;&gt;<span class="s">)</span> <span class="s">{</span></li><li>    <a class="l_k" href="functions/print.html">print</a> <a class="l_k" href="functions/if.html">if</a> <span class="q">/$arg/o</span><span class="sc">;</span> <span class="c"># compile only once (no longer needed!)</span></li><li> <span class="s">}</span></li><li></li><li> <a class="l_k" href="functions/if.html">if</a> <span class="s">(</span><span class="s">(</span><span class="i">$F1</span><span class="cm">,</span> <span class="i">$F2</span><span class="cm">,</span> <span class="i">$Etc</span><span class="s">)</span> = <span class="s">(</span><span class="i">$foo</span> =~ <span class="q">/^(\S+)\s+(\S+)\s*(.*)/</span><span class="s">)</span><span class="s">)</span></li></ol></pre><p>This last example splits <code class="inline"><span class="i">$foo</span></code>
 into the first two words and the
remainder of the line, and assigns those three fields to <code class="inline"><span class="i">$F1</span></code>
, <code class="inline"><span class="i">$F2</span></code>
, and
<code class="inline"><span class="i">$Etc</span></code>
.  The conditional is true if any variables were assigned; that is,
if the pattern matched.</p>
<p>The <code class="inline">/g</code> modifier specifies global pattern matching--that is,
matching as many times as possible within the string.  How it behaves
depends on the context.  In list context, it returns a list of the
substrings matched by any capturing parentheses in the regular
expression.  If there are no parentheses, it returns a list of all
the matched strings, as if there were parentheses around the whole
pattern.</p>
<p>In scalar context, each execution of <code class="inline"><a class="l_k" href="functions/m.html">m//g</a></code> finds the next match,
returning true if it matches, and false if there is no further match.
The position after the last match can be read or set using the <code class="inline"><a class="l_k" href="functions/pos.html">pos()</a></code>
function; see <a href="functions/pos.html">pos</a>.  A failed match normally resets the
search position to the beginning of the string, but you can avoid that
by adding the <code class="inline">/c</code> modifier (for example, <code class="inline"><a class="l_k" href="functions/m.html">m//gc</a></code>).  Modifying the target
string also resets the search position.</p>
</li>
<li><a name="%5cG-_assertion_"></a><b><code class="inline">\G <i>assertion</i></code></b>
<p>You can intermix <code class="inline"><a class="l_k" href="functions/m.html">m//g</a></code> matches with <code class="inline"><a class="l_k" href="functions/m.html">m/\G.../g</a></code>, where <code class="inline">\<span class="w">G</span></code>
 is a
zero-width assertion that matches the exact position where the
previous <code class="inline"><a class="l_k" href="functions/m.html">m//g</a></code>, if any, left off.  Without the <code class="inline">/g</code> modifier, the
<code class="inline">\<span class="w">G</span></code>
 assertion still anchors at <code class="inline"><a class="l_k" href="functions/pos.html">pos()</a></code> as it was at the start of
the operation (see <a href="functions/pos.html">pos</a>), but the match is of course only
attempted once.  Using <code class="inline">\<span class="w">G</span></code>
 without <code class="inline">/g</code> on a target string that has
not previously had a <code class="inline">/g</code> match applied to it is the same as using
the <code class="inline">\<span class="w">A</span></code>
 assertion to match the beginning of the string.  Note also
that, currently, <code class="inline">\<span class="w">G</span></code>
 is only properly supported when anchored at the
very beginning of the pattern.</p>
<p>Examples:</p>
<pre class="verbatim"><ol><li>    <span class="c"># list context</span></li><li>    <span class="s">(</span><span class="i">$one</span><span class="cm">,</span><span class="i">$five</span><span class="cm">,</span><span class="i">$fifteen</span><span class="s">)</span> = <span class="s">(</span><span class="q">`uptime`</span> =~ <span class="q">/(\d+\.\d+)/g</span><span class="s">)</span><span class="sc">;</span></li><li></li><li>    <span class="c"># scalar context</span></li><li>    <a class="l_k" href="functions/local.html">local</a> <span class="i">$/</span> = <span class="q">&quot;&quot;</span><span class="sc">;</span></li><li>    while <span class="s">(</span><span class="i">$paragraph</span> = &lt;&gt;<span class="s">)</span> <span class="s">{</span></li><li>	while <span class="s">(</span><span class="i">$paragraph</span> =~ <span class="q">/\p{Ll}[&#39;&quot;)]*[.!?]+[&#39;&quot;)]*\s/g</span><span class="s">)</span> <span class="s">{</span></li><li>	    <span class="i">$sentences</span>++<span class="sc">;</span></li><li>	<span class="s">}</span></li><li>    <span class="s">}</span></li><li>    <a class="l_k" href="functions/say.html">say</a> <span class="i">$sentences</span><span class="sc">;</span></li></ol></pre><p>Here's another way to check for sentences in a paragraph:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/my.html">my</a> <span class="i">$sentence_rx</span> = <span class="q">qr{</span></li><li>    <span class="q">    (?: (?&lt;= ^ ) | (?&lt;= \s ) )  # after start-of-string or</span></li><li>                                <span class="q">                                # whitespace</span></li><li>    <span class="q">    \p{Lu}                      # capital letter</span></li><li>    <span class="q">    .*?                         # a bunch of anything</span></li><li>    <span class="q">    (?&lt;= \S )                   # that ends in non-</span></li><li>                                <span class="q">                                # whitespace</span></li><li>    <span class="q">    (?&lt;! \b [DMS]r  )           # but isn&#39;t a common abbr.</span></li><li>    <span class="q">    (?&lt;! \b Mrs )</span></li><li>    <span class="q">    (?&lt;! \b Sra )</span></li><li>    <span class="q">    (?&lt;! \b St  )</span></li><li>    <span class="q">    [.?!]                       # followed by a sentence</span></li><li>                                <span class="q">                                # ender</span></li><li>    <span class="q">    (?= $ | \s )                # in front of end-of-string</span></li><li>                                <span class="q">                                # or whitespace</span></li><li> <span class="q"> }sx</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/local.html">local</a> <span class="i">$/</span> = <span class="q">&quot;&quot;</span><span class="sc">;</span></li><li> <a class="l_k" href="functions/while.html">while</a> <span class="s">(</span><a class="l_k" href="functions/my.html">my</a> <span class="i">$paragraph</span> = &lt;&gt;<span class="s">)</span> <span class="s">{</span></li><li>    <a class="l_k" href="functions/say.html">say</a> <span class="q">&quot;NEW PARAGRAPH&quot;</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/my.html">my</a> <span class="i">$count</span> = <span class="n">0</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/while.html">while</a> <span class="s">(</span><span class="i">$paragraph</span> =~ <span class="q">/($sentence_rx)/g</span><span class="s">)</span> <span class="s">{</span></li><li>        <a class="l_k" href="functions/printf.html">printf</a> <span class="q">&quot;\tgot sentence %d: &lt;%s&gt;\n&quot;</span><span class="cm">,</span> ++<span class="i">$count</span><span class="cm">,</span> <span class="i">$1</span><span class="sc">;</span></li><li>    <span class="s">}</span></li><li> <span class="s">}</span></li></ol></pre><p>Here's how to use <code class="inline"><a class="l_k" href="functions/m.html">m//gc</a></code> with <code class="inline">\<span class="w">G</span></code>
:</p>
<pre class="verbatim"><ol><li>    <span class="i">$_</span> = <span class="q">&quot;ppooqppqq&quot;</span><span class="sc">;</span></li><li>    while <span class="s">(</span><span class="i">$i</span>++ &lt; <span class="n">2</span><span class="s">)</span> <span class="s">{</span></li><li>        <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;1: &#39;&quot;</span><span class="sc">;</span></li><li>        <a class="l_k" href="functions/print.html">print</a> <span class="i">$1</span> while <span class="q">/(o)/gc</span><span class="sc">;</span> <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;&#39;, pos=&quot;</span><span class="cm">,</span> <a class="l_k" href="functions/pos.html">pos</a><span class="cm">,</span> <span class="q">&quot;\n&quot;</span><span class="sc">;</span></li><li>        <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;2: &#39;&quot;</span><span class="sc">;</span></li><li>        <a class="l_k" href="functions/print.html">print</a> <span class="i">$1</span> if <span class="q">/\G(q)/gc</span><span class="sc">;</span>  <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;&#39;, pos=&quot;</span><span class="cm">,</span> <a class="l_k" href="functions/pos.html">pos</a><span class="cm">,</span> <span class="q">&quot;\n&quot;</span><span class="sc">;</span></li><li>        <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;3: &#39;&quot;</span><span class="sc">;</span></li><li>        <a class="l_k" href="functions/print.html">print</a> <span class="i">$1</span> while <span class="q">/(p)/gc</span><span class="sc">;</span> <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;&#39;, pos=&quot;</span><span class="cm">,</span> <a class="l_k" href="functions/pos.html">pos</a><span class="cm">,</span> <span class="q">&quot;\n&quot;</span><span class="sc">;</span></li><li>    <span class="s">}</span></li><li>    <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;Final: &#39;$1&#39;, pos=&quot;</span><span class="cm">,</span><a class="l_k" href="functions/pos.html">pos</a><span class="cm">,</span><span class="q">&quot;\n&quot;</span> if <span class="q">/\G(.)/</span><span class="sc">;</span></li></ol></pre><p>The last example should print:</p>
<pre class="verbatim"><ol><li>    1: 'oo', pos=4</li><li>    2: 'q', pos=5</li><li>    3: 'pp', pos=7</li><li>    1: '', pos=7</li><li>    2: 'q', pos=8</li><li>    3: '', pos=8</li><li>    Final: 'q', pos=8</li></ol></pre><p>Notice that the final match matched <code class="inline"><a class="l_k" href="functions/q.html">q</a></code> instead of <code class="inline"><span class="w">p</span></code>
, which a match
without the <code class="inline">\<span class="w">G</span></code>
 anchor would have done.  Also note that the final match
did not update <code class="inline"><a class="l_k" href="functions/pos.html">pos</a></code>.  <code class="inline"><a class="l_k" href="functions/pos.html">pos</a></code> is only updated on a <code class="inline">/g</code> match.  If the
final match did indeed match <code class="inline"><span class="w">p</span></code>
, it's a good bet that you're running a
very old (pre-5.6.0) version of Perl.</p>
<p>A useful idiom for <code class="inline"><span class="w">lex</span></code>
-like scanners is <code class="inline"><span class="q">/\G.../gc</span></code>
.  You can
combine several regexps like this to process a string part-by-part,
doing different actions depending on which regexp matched.  Each
regexp tries to match where the previous one leaves off.</p>
<pre class="verbatim"><ol><li> <span class="i">$_</span> = <span class="h">&lt;&lt;&#39;EOL&#39;</span><span class="sc">;</span></li><li><span class="hh">    $url = URI::URL-&gt;new( &quot;<a href="http://example.com/">http://example.com/</a>&quot; );</span></li><li><span class="hh">    die if $url eq &quot;xXx&quot;;</span></li><li><span class="hh"> EOL</span></li><li></li><li><span class="hh"> LOOP: {</span></li><li><span class="hh">     print(&quot; digits&quot;),       redo LOOP if /\G\d+\b[,.;]?\s*/gc;</span></li><li><span class="hh">     print(&quot; lowercase&quot;),    redo LOOP</span></li><li><span class="hh">                                    if /\G\p{Ll}+\b[,.;]?\s*/gc;</span></li><li><span class="hh">     print(&quot; UPPERCASE&quot;),    redo LOOP</span></li><li><span class="hh">                                    if /\G\p{Lu}+\b[,.;]?\s*/gc;</span></li><li><span class="hh">     print(&quot; Capitalized&quot;),  redo LOOP</span></li><li><span class="hh">                              if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc;</span></li><li><span class="hh">     print(&quot; MiXeD&quot;),        redo LOOP if /\G\pL+\b[,.;]?\s*/gc;</span></li><li><span class="hh">     print(&quot; alphanumeric&quot;), redo LOOP</span></li><li><span class="hh">                            if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc;</span></li><li><span class="hh">     print(&quot; line-noise&quot;),   redo LOOP if /\G\W+/gc;</span></li><li><span class="hh">     print &quot;. That&#39;s all!\n&quot;;</span></li><li><span class="hh"> }</span></li></ol></pre><p>Here is the output (split into several lines):</p>
<pre class="verbatim"><ol><li> <span class="w">line</span>-<span class="w">noise</span> <span class="w">lowercase</span> <span class="w">line</span>-<span class="w">noise</span> <span class="w">UPPERCASE</span> <span class="w">line</span>-<span class="w">noise</span> <span class="w">UPPERCASE</span></li><li> <span class="w">line</span>-<span class="w">noise</span> <span class="w">lowercase</span> <span class="w">line</span>-<span class="w">noise</span> <span class="w">lowercase</span> <span class="w">line</span>-<span class="w">noise</span> <span class="w">lowercase</span></li><li> <span class="w">lowercase</span> <span class="w">line</span>-<span class="w">noise</span> <span class="w">lowercase</span> <span class="w">lowercase</span> <span class="w">line</span>-<span class="w">noise</span> <span class="w">lowercase</span></li><li> <span class="w">lowercase</span> <span class="w">line</span>-<span class="w">noise</span> <span class="w">MiXeD</span> <span class="w">line</span>-<span class="w">noise</span>. <span class="w">That&#39;s</span> <span class="w">all</span>!</li></ol></pre></li>
<li><a name="m%3f_PATTERN_%3fmsixpodualngc"></a><b><code class="inline">m?<i>PATTERN</i>?msixpodualngc</code>
 </b>
<p>This is just like the <code class="inline">m/<i>PATTERN</i>/</code> search, except that it matches
only once between calls to the <code class="inline"><a class="l_k" href="functions/reset.html">reset()</a></code> operator.  This is a useful
optimization when you want to see only the first occurrence of
something in each file of a set of files, for instance.  Only <code class="inline"><a class="l_k" href="functions/m.html">m??</a></code>
patterns local to the current package are reset.</p>
<pre class="verbatim"><ol><li>    while <span class="s">(</span>&lt;&gt;<span class="s">)</span> <span class="s">{</span></li><li>	if <span class="s">(</span><span class="q">m?^$?</span><span class="s">)</span> <span class="s">{</span></li><li>			    <span class="c"># blank line between header and body</span></li><li>	<span class="s">}</span></li><li>    <span class="s">}</span> <a class="l_k" href="functions/continue.html">continue</a> <span class="s">{</span></li><li>	<a class="l_k" href="functions/reset.html">reset</a> if <a class="l_k" href="functions/eof.html">eof</a><span class="sc">;</span>	    <span class="c"># clear m?? status for next file</span></li><li>    <span class="s">}</span></li></ol></pre><p>Another example switched the first "latin1" encoding it finds
to "utf8" in a pod file:</p>
<pre class="verbatim"><ol><li>    <span class="q">s//utf8/</span> if <span class="q">m? ^ =encoding \h+ \K latin1 ?x</span><span class="sc">;</span></li></ol></pre><p>The match-once behavior is controlled by the match delimiter being
<code class="inline">?</code>; with any other delimiter this is the normal <code class="inline"><a class="l_k" href="functions/m.html">m//</a></code> operator.</p>
<p>In the past, the leading <code class="inline"><a class="l_k" href="functions/m.html">m</a></code> in <code class="inline">m?<i>PATTERN</i>?</code> was optional, but omitting it
would produce a deprecation warning.  As of v5.22.0, omitting it produces a
syntax error.  If you encounter this construct in older code, you can just add
<code class="inline"><a class="l_k" href="functions/m.html">m</a></code>.</p>
</li>
<li><a name="s%2f_PATTERN_%2f_REPLACEMENT_%2fmsixpodualngcer"></a><b><code class="inline">s/<i>PATTERN</i>/<i>REPLACEMENT</i>/msixpodualngcer</code>
   
          </b>
<p>Searches a string for a pattern, and if found, replaces that pattern
with the replacement text and returns the number of substitutions
made.  Otherwise it returns false (specifically, the empty string).</p>
<p>If the <code class="inline"><span class="q">/r</span></code>
 (non-destructive) option is used then it runs the
substitution on a copy of the string and instead of returning the
number of substitutions, it returns the copy whether or not a
substitution occurred.  The original string is never changed when
<code class="inline"><span class="q">/r</span></code>
 is used.  The copy will always be a plain string, even if the
input is an object or a tied variable.</p>
<p>If no string is specified via the <code class="inline">=~</code>
 or <code class="inline">!~</code>
 operator, the <code class="inline"><span class="i">$_</span></code>

variable is searched and modified.  Unless the <code class="inline"><span class="q">/r</span></code>
 option is used,
the string specified must be a scalar variable, an array element, a
hash element, or an assignment to one of those; that is, some sort of
scalar lvalue.</p>
<p>If the delimiter chosen is a single quote, no variable interpolation is
done on either the <i>PATTERN</i> or the <i>REPLACEMENT</i>.  Otherwise, if the
<i>PATTERN</i> contains a <code class="inline"><span class="i">$</span></code>
 that looks like a variable rather than an
end-of-string test, the variable will be interpolated into the pattern
at run-time.  If you want the pattern compiled only once the first time
the variable is interpolated, use the <code class="inline">/o</code> option.  If the pattern
evaluates to the empty string, the last successfully executed regular
expression is used instead.  See <a href="perlre.html">perlre</a> for further explanation on these.</p>
<p>Options are as with <code class="inline"><a class="l_k" href="functions/m.html">m//</a></code> with the addition of the following replacement
specific options:</p>
<pre class="verbatim"><ol><li>    <span class="w">e</span>	<span class="w">Evaluate</span> <span class="w">the</span> <span class="w">right</span> <span class="w">side</span> <span class="w">as</span> <span class="w">an</span> <span class="w">expression</span>.</li><li>    <span class="w">ee</span>  <span class="w">Evaluate</span> <span class="w">the</span> <span class="w">right</span> <span class="w">side</span> <span class="w">as</span> <span class="w">a</span> <span class="w">string</span> <span class="w">then</span> <a class="l_k" href="functions/eval.html">eval</a> <span class="w">the</span></li><li>        <span class="w">result</span>.</li><li>    <span class="w">r</span>   <span class="w">Return</span> <span class="w">substitution</span> <a class="l_k" href="functions/and.html">and</a> <span class="w">leave</span> <span class="w">the</span> <span class="w">original</span> <span class="w">string</span></li><li>        <span class="w">untouched</span>.</li></ol></pre><p>Any non-whitespace delimiter may replace the slashes.  Add space after
the <code class="inline"><a class="l_k" href="functions/s.html">s</a></code> when using a character allowed in identifiers.  If single quotes
are used, no interpretation is done on the replacement string (the <code class="inline">/e</code>
modifier overrides this, however).  Note that Perl treats backticks
as normal delimiters; the replacement text is not evaluated as a command.
If the <i>PATTERN</i> is delimited by bracketing quotes, the <i>REPLACEMENT</i> has
its own pair of quotes, which may or may not be bracketing quotes, for example,
<code class="inline"><a class="l_k" href="functions/s.html">s(foo)(bar)</a></code> or <code class="inline"><a class="l_k" href="functions/s.html">s<foo>/bar/</a></code>.  A <code class="inline">/e</code> will cause the
replacement portion to be treated as a full-fledged Perl expression
and evaluated right then and there.  It is, however, syntax checked at
compile-time.  A second <code class="inline"><span class="w">e</span></code>
 modifier will cause the replacement portion
to be <code class="inline"><a class="l_k" href="functions/eval.html">eval</a></code>ed before being run as a Perl expression.</p>
<p>Examples:</p>
<pre class="verbatim"><ol><li>    <span class="q">s/\bgreen\b/mauve/g</span><span class="sc">;</span>	      <span class="c"># don&#39;t change wintergreen</span></li><li></li><li>    <span class="i">$path</span> =~ <span class="q">s|/usr/bin|/usr/local/bin|</span><span class="sc">;</span></li><li></li><li>    <span class="q">s/Login: $foo/Login: $bar/</span><span class="sc">;</span> <span class="c"># run-time pattern</span></li><li></li><li>    <span class="s">(</span><span class="i">$foo</span> = <span class="i">$bar</span><span class="s">)</span> =~ <span class="q">s/this/that/</span><span class="sc">;</span>	<span class="c"># copy first, then</span></li><li>                                        <span class="c"># change</span></li><li>    <span class="s">(</span><span class="i">$foo</span> = <span class="q">&quot;$bar&quot;</span><span class="s">)</span> =~ <span class="q">s/this/that/</span><span class="sc">;</span>	<span class="c"># convert to string,</span></li><li>                                        <span class="c"># copy, then change</span></li><li>    <span class="i">$foo</span> = <span class="i">$bar</span> =~ <span class="q">s/this/that/</span><span class="w">r</span><span class="sc">;</span>	<span class="c"># Same as above using /r</span></li><li>    <span class="i">$foo</span> = <span class="i">$bar</span> =~ <span class="q">s/this/that/</span><span class="w">r</span></li><li>                =~ <span class="q">s/that/the other/</span><span class="w">r</span><span class="sc">;</span>	<span class="c"># Chained substitutes</span></li><li>                                        <span class="c"># using /r</span></li><li>    <span class="i">@foo</span> = <a class="l_k" href="functions/map.html">map</a> <span class="s">{</span> <span class="q">s/this/that/</span><span class="w">r</span> <span class="s">}</span> <span class="i">@bar</span>	<span class="c"># /r is very useful in</span></li><li>                                        <span class="c"># maps</span></li><li></li><li>    <span class="i">$count</span> = <span class="s">(</span><span class="i">$paragraph</span> =~ <span class="q">s/Mister\b/Mr./g</span><span class="s">)</span><span class="sc">;</span>  <span class="c"># get change-cnt</span></li><li></li><li>    <span class="i">$_</span> = <span class="q">&#39;abc123xyz&#39;</span><span class="sc">;</span></li><li>    <span class="q">s/\d+/$&amp;*2/e</span><span class="sc">;</span>		<span class="c"># yields &#39;abc246xyz&#39;</span></li><li>    <span class="q">s/\d+/sprintf(&quot;%5d&quot;,$&amp;)/e</span><span class="sc">;</span>	<span class="c"># yields &#39;abc  246xyz&#39;</span></li><li>    <span class="q">s/\w/$&amp; x 2/eg</span><span class="sc">;</span>		<span class="c"># yields &#39;aabbcc  224466xxyyzz&#39;</span></li><li></li><li>    <span class="q">s/%(.)/$percent{$1}/g</span><span class="sc">;</span>	<span class="c"># change percent escapes; no /e</span></li><li>    <span class="q">s/%(.)/$percent{$1} || $&amp;/ge</span><span class="sc">;</span>	<span class="c"># expr now, so /e</span></li><li>    <span class="q">s/^=(\w+)/pod($1)/ge</span><span class="sc">;</span>	<span class="c"># use function call</span></li><li></li><li>    <span class="i">$_</span> = <span class="q">&#39;abc123xyz&#39;</span><span class="sc">;</span></li><li>    <span class="i">$x</span> = <span class="q">s/abc/def/</span><span class="w">r</span><span class="sc">;</span>           <span class="c"># $x is &#39;def123xyz&#39; and</span></li><li>                                <span class="c"># $_ remains &#39;abc123xyz&#39;.</span></li><li></li><li>    <span class="c"># expand variables in $_, but dynamics only, using</span></li><li>    <span class="c"># symbolic dereferencing</span></li><li>    <span class="q">s/\$(\w+)/${$1}/g</span><span class="sc">;</span></li><li></li><li>    <span class="c"># Add one to the value of any numbers in the string</span></li><li>    <span class="q">s/(\d+)/1 + $1/eg</span><span class="sc">;</span></li><li></li><li>    <span class="c"># Titlecase words in the last 30 characters only</span></li><li>    <a class="l_k" href="functions/substr.html">substr</a><span class="s">(</span><span class="i">$str</span><span class="cm">,</span> <span class="n">-30</span><span class="s">)</span> =~ <span class="q">s/\b(\p{Alpha}+)\b/\u\L$1/g</span><span class="sc">;</span></li><li></li><li>    <span class="c"># This will expand any embedded scalar variable</span></li><li>    <span class="c"># (including lexicals) in $_ : First $1 is interpolated</span></li><li>    <span class="c"># to the variable name, and then evaluated</span></li><li>    <span class="q">s/(\$\w+)/$1/eeg</span><span class="sc">;</span></li><li></li><li>    <span class="c"># Delete (most) C comments.</span></li><li>    <span class="i">$program</span> =~ <span class="q">s {</span></li><li>	<span class="q">	/\*	# Match the opening delimiter.</span></li><li>	<span class="q">	.*?	# Match a minimal number of characters.</span></li><li>	<span class="q">	\*/	# Match the closing delimiter.</span></li><li>    <span class="q">    } []gsx</span><span class="sc">;</span></li><li></li><li>    <span class="q">s/^\s*(.*?)\s*$/$1/</span><span class="sc">;</span>	<span class="c"># trim whitespace in $_,</span></li><li>                                <span class="c"># expensively</span></li><li></li><li>    <a class="l_k" href="functions/for.html">for</a> <span class="s">(</span><span class="i">$variable</span><span class="s">)</span> <span class="s">{</span>		<span class="c"># trim whitespace in $variable,</span></li><li>                                <span class="c"># cheap</span></li><li>	<span class="q">s/^\s+//</span><span class="sc">;</span></li><li>	<span class="q">s/\s+$//</span><span class="sc">;</span></li><li>    <span class="s">}</span></li><li></li><li>    <span class="q">s/([^ ]*) *([^ ]*)/$2 $1/</span><span class="sc">;</span>	<span class="c"># reverse 1st two fields</span></li></ol></pre><p>Note the use of <code class="inline"><span class="i">$</span></code>
 instead of <code class="inline">\</code>
 in the last example.  Unlike
<b>sed</b>, we use the \&lt;<i>digit</i>&gt; form only in the left hand side.
Anywhere else it's $&lt;<i>digit</i>&gt;.</p>
<p>Occasionally, you can't use just a <code class="inline">/g</code> to get all the changes
to occur that you might want.  Here are two common cases:</p>
<pre class="verbatim"><ol><li>    <span class="c"># put commas in the right places in an integer</span></li><li>    <span class="n">1</span> while <span class="q">s/(\d)(\d\d\d)(?!\d)/$1,$2/g</span><span class="sc">;</span></li><li></li><li>    <span class="c"># expand tabs to 8-column spacing</span></li><li>    <span class="n">1</span> while <span class="q">s/\t+/&#39; &#39; x (length($&amp;)*8 - length($`)%8)/e</span><span class="sc">;</span></li></ol></pre></li>
</ul>
<a name="Quote-Like-Operators"></a><h2>Quote-Like Operators
</h2>
<ul>
<li><a name="q%2f_STRING_%2f"></a><b><code class="inline">q/<i>STRING</i>/</code>
   </b>
</li>
<li><a name="'_STRING_'"></a><b><code class="inline">'<i>STRING</i>'</code></b>
<p>A single-quoted, literal string.  A backslash represents a backslash
unless followed by the delimiter or another backslash, in which case
the delimiter or backslash is interpolated.</p>
<pre class="verbatim"><ol><li>    <span class="i">$foo</span> = <span class="q">q!I said, &quot;You said, &#39;She said it.&#39;&quot;!</span><span class="sc">;</span></li><li>    <span class="i">$bar</span> = <span class="q">q(&#39;This is it.&#39;)</span><span class="sc">;</span></li><li>    <span class="i">$baz</span> = <span class="q">&#39;\n&#39;</span><span class="sc">;</span>		<span class="c"># a two-character string</span></li></ol></pre></li>
<li><a name="qq%2f_STRING_%2f"></a><b><code class="inline">qq/<i>STRING</i>/</code>
   </b>
</li>
<li><a name="%22_STRING_%22"></a><b>"<i>STRING</i>"</b>
<p>A double-quoted, interpolated string.</p>
<pre class="verbatim"><ol><li>    <span class="i">$_</span> .= <span class="q">qq</span></li><li>     <span class="q">     (*** The previous line contains the naughty word &quot;$1&quot;.\n)</span></li><li>		if <span class="q">/\b(tcl|java|python)\b/i</span><span class="sc">;</span>      <span class="c"># :-)</span></li><li>    <span class="i">$baz</span> = <span class="q">&quot;\n&quot;</span><span class="sc">;</span>		<span class="c"># a one-character string</span></li></ol></pre></li>
<li><a name="qx%2f_STRING_%2f"></a><b><code class="inline">qx/<i>STRING</i>/</code>
   </b>
</li>
<li><a name="%60_STRING_%60"></a><b><code class="inline">`<i>STRING</i>`</code></b>
<p>A string which is (possibly) interpolated and then executed as a
system command with <i>/bin/sh</i> or its equivalent.  Shell wildcards,
pipes, and redirections will be honored.  The collected standard
output of the command is returned; standard error is unaffected.  In
scalar context, it comes back as a single (potentially multi-line)
string, or <code class="inline"><a class="l_k" href="functions/undef.html">undef</a></code> if the command failed.  In list context, returns a
list of lines (however you've defined lines with <code class="inline"><span class="i">$/</span></code>
 or
<code class="inline"><span class="i">$INPUT_RECORD_SEPARATOR</span></code>
), or an empty list if the command failed.</p>
<p>Because backticks do not affect standard error, use shell file descriptor
syntax (assuming the shell supports this) if you care to address this.
To capture a command's STDERR and STDOUT together:</p>
<pre class="verbatim"><ol><li>    <span class="i">$output</span> = <span class="q">`cmd 2&gt;&amp;1`</span><span class="sc">;</span></li></ol></pre><p>To capture a command's STDOUT but discard its STDERR:</p>
<pre class="verbatim"><ol><li>    <span class="i">$output</span> = <span class="q">`cmd 2&gt;/dev/null`</span><span class="sc">;</span></li></ol></pre><p>To capture a command's STDERR but discard its STDOUT (ordering is
important here):</p>
<pre class="verbatim"><ol><li>    <span class="i">$output</span> = <span class="q">`cmd 2&gt;&amp;1 1&gt;/dev/null`</span><span class="sc">;</span></li></ol></pre><p>To exchange a command's STDOUT and STDERR in order to capture the STDERR
but leave its STDOUT to come out the old STDERR:</p>
<pre class="verbatim"><ol><li>    <span class="i">$output</span> = <span class="q">`cmd 3&gt;&amp;1 1&gt;&amp;2 2&gt;&amp;3 3&gt;&amp;-`</span><span class="sc">;</span></li></ol></pre><p>To read both a command's STDOUT and its STDERR separately, it's easiest
to redirect them separately to files, and then read from those files
when the program is done:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/system.html">system</a><span class="s">(</span><span class="q">&quot;program args 1&gt;program.stdout 2&gt;program.stderr&quot;</span><span class="s">)</span><span class="sc">;</span></li></ol></pre><p>The STDIN filehandle used by the command is inherited from Perl's STDIN.
For example:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/open.html">open</a><span class="s">(</span><span class="w">SPLAT</span><span class="cm">,</span> <span class="q">&quot;stuff&quot;</span><span class="s">)</span>   || <a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;can&#39;t open stuff: $!&quot;</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/open.html">open</a><span class="s">(</span><span class="w">STDIN</span><span class="cm">,</span> <span class="q">&quot;&lt;&amp;SPLAT&quot;</span><span class="s">)</span> || <a class="l_k" href="functions/die.html">die</a> <span class="q">&quot;can&#39;t dupe SPLAT: $!&quot;</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/print.html">print</a> <span class="i">STDOUT</span> <span class="q">`sort`</span><span class="sc">;</span></li></ol></pre><p>will print the sorted contents of the file named <i>"stuff"</i>.</p>
<p>Using single-quote as a delimiter protects the command from Perl's
double-quote interpolation, passing it on to the shell instead:</p>
<pre class="verbatim"><ol><li>    <span class="i">$perl_info</span>  = <span class="q">qx(ps $$)</span><span class="sc">;</span>            <span class="c"># that&#39;s Perl&#39;s $$</span></li><li>    <span class="i">$shell_info</span> = <span class="q">qx&#39;ps $$&#39;</span><span class="sc">;</span>            <span class="c"># that&#39;s the new shell&#39;s $$</span></li></ol></pre><p>How that string gets evaluated is entirely subject to the command
interpreter on your system.  On most platforms, you will have to protect
shell metacharacters if you want them treated literally.  This is in
practice difficult to do, as it's unclear how to escape which characters.
See <a href="perlsec.html">perlsec</a> for a clean and safe example of a manual <code class="inline"><a class="l_k" href="functions/fork.html">fork()</a></code> and <code class="inline"><a class="l_k" href="functions/exec.html">exec()</a></code>
to emulate backticks safely.</p>
<p>On some platforms (notably DOS-like ones), the shell may not be
capable of dealing with multiline commands, so putting newlines in
the string may not get you what you want.  You may be able to evaluate
multiple commands in a single line by separating them with the command
separator character, if your shell supports that (for example, <code class="inline"><span class="sc">;</span></code>
 on
many Unix shells and <code class="inline"><span class="i">&amp;</span></code>
 on the Windows NT <code class="inline"><span class="w">cmd</span></code>
 shell).</p>
<p>Perl will attempt to flush all files opened for
output before starting the child process, but this may not be supported
on some platforms (see <a href="perlport.html">perlport</a>).  To be safe, you may need to set
<code class="inline"><span class="i">$|</span></code>
 (<code class="inline"><span class="i">$AUTOFLUSH</span></code>
 in <code class="inline"><a href="English.html">English</a></code>) or call the <code class="inline"><span class="i">autoflush</span><span class="s">(</span><span class="s">)</span></code>
 method of
<code class="inline"><a href="IO/Handle.html">IO::Handle</a></code> on any open handles.</p>
<p>Beware that some command shells may place restrictions on the length
of the command line.  You must ensure your strings don't exceed this
limit after any necessary interpolations.  See the platform-specific
release notes for more details about your particular environment.</p>
<p>Using this operator can lead to programs that are difficult to port,
because the shell commands called vary between systems, and may in
fact not be present at all.  As one example, the <code class="inline"><span class="w">type</span></code>
 command under
the POSIX shell is very different from the <code class="inline"><span class="w">type</span></code>
 command under DOS.
That doesn't mean you should go out of your way to avoid backticks
when they're the right way to get something done.  Perl was made to be
a glue language, and one of the things it glues together is commands.
Just understand what you're getting yourself into.</p>
<p>Like <code class="inline"><a class="l_k" href="functions/system.html">system</a></code>, backticks put the child process exit code in <code class="inline"><span class="i">$?</span></code>
.
If you'd like to manually inspect failure, you can check all possible
failure modes by inspecting <code class="inline"><span class="i">$?</span></code>
 like this:</p>
<pre class="verbatim"><ol><li>    if <span class="s">(</span><span class="i">$?</span> == <span class="n">-1</span><span class="s">)</span> <span class="s">{</span></li><li>        <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;failed to execute: $!\n&quot;</span><span class="sc">;</span></li><li>    <span class="s">}</span></li><li>    elsif <span class="s">(</span><span class="i">$?</span> &amp; <span class="n">127</span><span class="s">)</span> <span class="s">{</span></li><li>        <a class="l_k" href="functions/printf.html">printf</a> <span class="q">&quot;child died with signal %d, %s coredump\n&quot;</span><span class="cm">,</span></li><li>            <span class="s">(</span><span class="i">$?</span> &amp; <span class="n">127</span><span class="s">)</span><span class="cm">,</span>  <span class="s">(</span><span class="i">$?</span> &amp; <span class="n">128</span><span class="s">)</span> ? <span class="q">&#39;with&#39;</span> <span class="co">:</span> <span class="q">&#39;without&#39;</span><span class="sc">;</span></li><li>    <span class="s">}</span></li><li>    else <span class="s">{</span></li><li>        <a class="l_k" href="functions/printf.html">printf</a> <span class="q">&quot;child exited with value %d\n&quot;</span><span class="cm">,</span> <span class="i">$?</span> &gt;&gt; <span class="n">8</span><span class="sc">;</span></li><li>    <span class="s">}</span></li></ol></pre><p>Use the <a href="open.html">open</a> pragma to control the I/O layers used when reading the
output of the command, for example:</p>
<pre class="verbatim"><ol><li>  <a class="l_k" href="functions/use.html">use</a> <a class="l_k" href="functions/open.html">open</a> <span class="w">IN</span> <span class="cm">=&gt;</span> <span class="q">&quot;:encoding(UTF-8)&quot;</span><span class="sc">;</span></li><li>  <a class="l_k" href="functions/my.html">my</a> <span class="i">$x</span> = <span class="q">`cmd-producing-utf-8`</span><span class="sc">;</span></li></ol></pre><p>See <a href="#I%2fO-Operators">I/O Operators</a> for more discussion.</p>
</li>
<li><a name="qw%2f_STRING_%2f"></a><b><code class="inline">qw/<i>STRING</i>/</code>
  </b>
<p>Evaluates to a list of the words extracted out of <i>STRING</i>, using embedded
whitespace as the word delimiters.  It can be understood as being roughly
equivalent to:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/split.html">split</a><span class="s">(</span><span class="q">&quot; &quot;</span><span class="cm">,</span> <span class="q">q/STRING/</span><span class="s">)</span><span class="sc">;</span></li></ol></pre><p>the differences being that it generates a real list at compile time, and
in scalar context it returns the last element in the list.  So
this expression:</p>
<pre class="verbatim"><ol><li>    <span class="q">qw(foo bar baz)</span></li></ol></pre><p>is semantically equivalent to the list:</p>
<pre class="verbatim"><ol><li>    <span class="q">&quot;foo&quot;</span><span class="cm">,</span> <span class="q">&quot;bar&quot;</span><span class="cm">,</span> <span class="q">&quot;baz&quot;</span></li></ol></pre><p>Some frequently seen examples:</p>
<pre class="verbatim"><ol><li>    use POSIX qw( setlocale localeconv )</li><li>    @EXPORT = qw( foo bar baz );</li></ol></pre><p>A common mistake is to try to separate the words with commas or to
put comments into a multi-line <code class="inline"><a class="l_k" href="functions/qw.html">qw</a></code>-string.  For this reason, the
<code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">warnings</span></code>
 pragma and the <b>-w</b> switch (that is, the <code class="inline"><span class="i">$^W</span></code>
 variable)
produces warnings if the <i>STRING</i> contains the <code class="inline"><span class="q">&quot;,&quot;</span></code>
 or the <code class="inline"><span class="q">&quot;#&quot;</span></code>
 character.</p>
</li>
<li><a name="tr%2f_SEARCHLIST_%2f_REPLACEMENTLIST_%2fcdsr"></a><b><code class="inline">tr/<i>SEARCHLIST</i>/<i>REPLACEMENTLIST</i>/cdsr</code>
     </b>
</li>
<li><a name="y%2f_SEARCHLIST_%2f_REPLACEMENTLIST_%2fcdsr"></a><b><code class="inline">y/<i>SEARCHLIST</i>/<i>REPLACEMENTLIST</i>/cdsr</code></b>
<p>Transliterates all occurrences of the characters found in the search list
with the corresponding character in the replacement list.  It returns
the number of characters replaced or deleted.  If no string is
specified via the <code class="inline">=~</code>
 or <code class="inline">!~</code>
 operator, the <code class="inline"><span class="i">$_</span></code>
 string is transliterated.</p>
<p>If the <code class="inline"><span class="q">/r</span></code>
 (non-destructive) option is present, a new copy of the string
is made and its characters transliterated, and this copy is returned no
matter whether it was modified or not: the original string is always
left unchanged.  The new copy is always a plain string, even if the input
string is an object or a tied variable.</p>
<p>Unless the <code class="inline"><span class="q">/r</span></code>
 option is used, the string specified with <code class="inline">=~</code>
 must be a
scalar variable, an array element, a hash element, or an assignment to one
of those; in other words, an lvalue.</p>
<p>A character range may be specified with a hyphen, so <code class="inline"><a class="l_k" href="functions/tr.html">tr/A-J/0-9/</a></code>
does the same replacement as <code class="inline"><a class="l_k" href="functions/tr.html">tr/ACEGIBDFHJ/0246813579/</a></code>.
For <b>sed</b> devotees, <code class="inline"><a class="l_k" href="functions/y.html">y</a></code> is provided as a synonym for <code class="inline"><a class="l_k" href="functions/tr.html">tr</a></code>.  If the
<i>SEARCHLIST</i> is delimited by bracketing quotes, the <i>REPLACEMENTLIST</i>
must have its own pair of quotes, which may or may not be bracketing
quotes; for example, <code class="inline"><a class="l_k" href="functions/tr.html">tr[aeiouy][yuoiea]</a></code> or <code class="inline"><a class="l_k" href="functions/tr.html">tr(+\-*/)/ABCD/</a></code>.</p>
<p>Characters may be literals or any of the escape sequences accepted in
double-quoted strings.  But there is no variable interpolation, so <code class="inline"><span class="q">&quot;$&quot;</span></code>

and <code class="inline"><span class="q">&quot;@&quot;</span></code>
 are treated as literals.  A hyphen at the beginning or end, or
preceded by a backslash is considered a literal.  Escape sequence
details are in <a href="#Quote-and-Quote-like-Operators">the table near the beginning of this section</a>.</p>
<p>Note that <code class="inline"><a class="l_k" href="functions/tr.html">tr</a></code> does <b>not</b> do regular expression character classes such as
<code class="inline">\<span class="w">d</span></code>
 or <code class="inline">\<span class="w">pL</span></code>
.  The <code class="inline"><a class="l_k" href="functions/tr.html">tr</a></code> operator is not equivalent to the <code class="inline"><i>tr(1)</i></code>
utility.  <code class="inline"><a class="l_k" href="functions/tr.html">tr[a-z][A-Z]</a></code> will uppercase the 26 letters "a" through "z",
but for case changing not confined to ASCII, use
<a href="functions/lc.html">lc</a>, <a href="functions/uc.html">uc</a>,
<a href="functions/lcfirst.html">lcfirst</a>, <a href="functions/ucfirst.html">ucfirst</a>
(all documented in <a href="perlfunc.html">perlfunc</a>), or the
<a href="#s%2fPATTERN%2fREPLACEMENT%2fmsixpodualngcer">substitution operator s/PATTERN/REPLACEMENT/</a>
(with <code class="inline">\<span class="w">U</span></code>
, <code class="inline">\<span class="w">u</span></code>
, <code class="inline">\<span class="w">L</span></code>
, and <code class="inline">\<span class="w">l</span></code>
 string-interpolation escapes in the
<i>REPLACEMENT</i> portion).</p>
<p>Most ranges are unportable between character sets, but certain ones
signal Perl to do special handling to make them portable.  There are two
classes of portable ranges.  The first are any subsets of the ranges
<code class="inline"><span class="w">A</span>-<span class="w">Z</span></code>
, <code class="inline"><span class="w">a</span>-z</code>
, and <code class="inline"><span class="n">0</span>-<span class="n">9</span></code>
, when expressed as literal characters.</p>
<pre class="verbatim"><ol><li>  <span class="q">tr/h-k/H-K/</span></li></ol></pre><p>capitalizes the letters <code class="inline"><span class="q">&quot;h&quot;</span></code>
, <code class="inline"><span class="q">&quot;i&quot;</span></code>
, <code class="inline"><span class="q">&quot;j&quot;</span></code>
, and <code class="inline"><span class="q">&quot;k&quot;</span></code>
 and nothing
else, no matter what the platform's character set is.  In contrast, all
of</p>
<pre class="verbatim"><ol><li>  <span class="q">tr/\x68-\x6B/\x48-\x4B/</span></li><li>  <span class="q">tr/h-\x6B/H-\x4B/</span></li><li>  <span class="q">tr/\x68-k/\x48-K/</span></li></ol></pre><p>do the same capitalizations as the previous example when run on ASCII
platforms, but something completely different on EBCDIC ones.</p>
<p>The second class of portable ranges is invoked when one or both of the
range's end points are expressed as <code class="inline">\<span class="i">N</span><span class="s">{</span>...<span class="s">}</span></code>
</p>
<pre class="verbatim"><ol><li> <span class="i">$string</span> =~ <span class="q">tr/\N{U+20}-\N{U+7E}//d</span><span class="sc">;</span></li></ol></pre><p>removes from <code class="inline"><span class="i">$string</span></code>
 all the platform's characters which are
equivalent to any of Unicode U+0020, U+0021, ... U+007D, U+007E.  This
is a portable range, and has the same effect on every platform it is
run on.  It turns out that in this example, these are the ASCII
printable characters.  So after this is run, <code class="inline"><span class="i">$string</span></code>
 has only
controls and characters which have no ASCII equivalents.</p>
<p>But, even for portable ranges, it is not generally obvious what is
included without having to look things up.  A sound principle is to use
only ranges that begin from and end at either ASCII alphabetics of equal
case (<code class="inline"><span class="w">b</span>-e</code>
, <code class="inline"><span class="w">B</span>-<span class="w">E</span></code>
), or digits (<code class="inline"><span class="n">1</span>-<span class="n">4</span></code>
).  Anything else is unclear
(and unportable unless <code class="inline">\<span class="i">N</span><span class="s">{</span>...<span class="s">}</span></code>
 is used).  If in doubt, spell out the
character sets in full.</p>
<p>Options:</p>
<pre class="verbatim"><ol><li>    <span class="w">c</span>	<span class="w">Complement</span> <span class="w">the</span> <span class="w">SEARCHLIST</span>.</li><li>    <span class="w">d</span>	<span class="w">Delete</span> <span class="w">found</span> <span class="w">but</span> <span class="w">unreplaced</span> <span class="w">characters</span>.</li><li>    <span class="q">s	Squash duplicate replaced characters.</span></li><li>    <span class="q">    r	Return the modified string and leave the original string</span></li><li>	<span class="q">	untouched.</span></li></ol></pre><p>If the <code class="inline">/c</code> modifier is specified, the <i>SEARCHLIST</i> character set
is complemented.  If the <code class="inline">/d</code> modifier is specified, any characters
specified by <i>SEARCHLIST</i> not found in <i>REPLACEMENTLIST</i> are deleted.
(Note that this is slightly more flexible than the behavior of some
<b>tr</b> programs, which delete anything they find in the <i>SEARCHLIST</i>,
period.)  If the <code class="inline">/s</code> modifier is specified, sequences of characters
that were transliterated to the same character are squashed down
to a single instance of the character.</p>
<p>If the <code class="inline">/d</code> modifier is used, the <i>REPLACEMENTLIST</i> is always interpreted
exactly as specified.  Otherwise, if the <i>REPLACEMENTLIST</i> is shorter
than the <i>SEARCHLIST</i>, the final character is replicated till it is long
enough.  If the <i>REPLACEMENTLIST</i> is empty, the <i>SEARCHLIST</i> is replicated.
This latter is useful for counting characters in a class or for
squashing character sequences in a class.</p>
<p>Examples:</p>
<pre class="verbatim"><ol><li>    <span class="i">$ARGV</span>[<span class="n">1</span>] =~ <span class="q">tr/A-Z/a-z/</span><span class="sc">;</span>	<span class="c"># canonicalize to lower case ASCII</span></li><li></li><li>    <span class="i">$cnt</span> = <span class="q">tr/*/*/</span><span class="sc">;</span>		<span class="c"># count the stars in $_</span></li><li></li><li>    <span class="i">$cnt</span> = <span class="i">$sky</span> =~ <span class="q">tr/*/*/</span><span class="sc">;</span>	<span class="c"># count the stars in $sky</span></li><li></li><li>    <span class="i">$cnt</span> = <span class="q">tr/0-9//</span><span class="sc">;</span>		<span class="c"># count the digits in $_</span></li><li></li><li>    <span class="q">tr/a-zA-Z//s</span><span class="sc">;</span>		<span class="c"># bookkeeper -&gt; bokeper</span></li><li></li><li>    <span class="s">(</span><span class="i">$HOST</span> = <span class="i">$host</span><span class="s">)</span> =~ <span class="q">tr/a-z/A-Z/</span><span class="sc">;</span></li><li>     <span class="i">$HOST</span> = <span class="i">$host</span>  =~ <span class="q">tr/a-z/A-Z/</span><span class="w">r</span><span class="sc">;</span>   <span class="c"># same thing</span></li><li></li><li>    <span class="i">$HOST</span> = <span class="i">$host</span> =~ <span class="q">tr/a-z/A-Z/</span><span class="w">r</span>    <span class="c"># chained with s///r</span></li><li>                  =~ <span class="q">s/:/ -p/</span><span class="w">r</span><span class="sc">;</span></li><li></li><li>    <span class="q">tr/a-zA-Z/ /cs</span><span class="sc">;</span>		<span class="c"># change non-alphas to single space</span></li><li></li><li>    <span class="i">@stripped</span> = <a class="l_k" href="functions/map.html">map</a> <span class="q">tr/a-zA-Z/ /csr</span><span class="cm">,</span> <span class="i">@original</span><span class="sc">;</span></li><li>				<span class="c"># /r with map</span></li><li></li><li>    <span class="q">tr [\200-\377]</span></li><li>       <span class="q">       [\000-\177]</span><span class="sc">;</span>		<span class="c"># wickedly delete 8th bit</span></li></ol></pre><p>If multiple transliterations are given for a character, only the
first one is used:</p>
<pre class="verbatim"><ol><li>    <span class="q">tr/AAA/XYZ/</span></li></ol></pre><p>will transliterate any A to X.</p>
<p>Because the transliteration table is built at compile time, neither
the <i>SEARCHLIST</i> nor the <i>REPLACEMENTLIST</i> are subjected to double quote
interpolation.  That means that if you want to use variables, you
must use an <code class="inline"><a class="l_k" href="functions/eval.html">eval()</a></code>:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/eval.html">eval</a> <span class="q">&quot;tr/$oldlist/$newlist/&quot;</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/die.html">die</a> <span class="i">$@</span> if <span class="i">$@</span><span class="sc">;</span></li><li></li><li>    <a class="l_k" href="functions/eval.html">eval</a> <span class="q">&quot;tr/$oldlist/$newlist/, 1&quot;</span> or <a class="l_k" href="functions/die.html">die</a> <span class="i">$@</span><span class="sc">;</span></li></ol></pre></li>
<li><a name="%3c%3c_EOF_"></a><b><code class="inline">&lt;&lt;<i>EOF</i></code>
   </b>
<p>A line-oriented form of quoting is based on the shell "here-document"
syntax.  Following a <code class="inline">&lt;&lt;</code>
 you specify a string to terminate
the quoted material, and all lines following the current line down to
the terminating string are the value of the item.</p>
<p>Prefixing the terminating string with a <code class="inline">~</code>
 specifies that you
want to use <a href="#Indented-Here-docs">Indented Here-docs</a> (see below).</p>
<p>The terminating string may be either an identifier (a word), or some
quoted text.  An unquoted identifier works like double quotes.
There may not be a space between the <code class="inline">&lt;&lt;</code>
 and the identifier,
unless the identifier is explicitly quoted.  (If you put a space it
will be treated as a null identifier, which is valid, and matches the
first empty line.)  The terminating string must appear by itself
(unquoted and with no surrounding whitespace) on the terminating line.</p>
<p>If the terminating string is quoted, the type of quotes used determine
the treatment of the text.</p>
<ul>
<li><a name="Double-Quotes"></a><b>Double Quotes</b>
<p>Double quotes indicate that the text will be interpolated using exactly
the same rules as normal double quoted strings.</p>
<pre class="verbatim"><ol><li>       print &lt;&lt;EOF;</li><li>    The price is $Price.</li><li>    EOF</li><li></li><li>       print &lt;&lt; "EOF"; # same as above</li><li>    The price is $Price.</li><li>    EOF</li></ol></pre></li>
<li><a name="Single-Quotes"></a><b>Single Quotes</b>
<p>Single quotes indicate the text is to be treated literally with no
interpolation of its content.  This is similar to single quoted
strings except that backslashes have no special meaning, with <code class="inline">\\</code>

being treated as two backslashes and not one as they would in every
other quoting construct.</p>
<p>Just as in the shell, a backslashed bareword following the <code class="inline">&lt;&lt;</code>

means the same thing as a single-quoted string does:</p>
<pre class="verbatim"><ol><li>	<span class="i">$cost</span> = <span class="h">&lt;&lt;&#39;VISTA&#39;</span><span class="sc">;</span>  <span class="c"># hasta la ...</span></li><li><span class="hh">    That&#39;ll be $10 please, ma&#39;am.</span></li><li><span class="hh">    VISTA</span></li><li></li><li><span class="hh">	$cost = &lt;&lt;\VISTA;   # Same thing!</span></li><li><span class="hh">    That&#39;ll be $10 please, ma&#39;am.</span></li><li><span class="hh">    VISTA</span></li></ol></pre><p>This is the only form of quoting in perl where there is no need
to worry about escaping content, something that code generators
can and do make good use of.</p>
</li>
<li><a name="Backticks"></a><b>Backticks</b>
<p>The content of the here doc is treated just as it would be if the
string were embedded in backticks.  Thus the content is interpolated
as though it were double quoted and then executed via the shell, with
the results of the execution returned.</p>
<pre class="verbatim"><ol><li>       print &lt;&lt; `EOC`; # execute command and get results</li><li>    echo hi there</li><li>    EOC</li></ol></pre></li>
</ul>
<ul>
<li><a name="Indented-Here-docs"></a><b>Indented Here-docs</b>
<p>The here-doc modifier <code class="inline">~</code>
 allows you to indent your here-docs to make
the code more readable:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/if.html">if</a> <span class="s">(</span><span class="i">$some_var</span><span class="s">)</span> <span class="s">{</span></li><li>      <a class="l_k" href="functions/print.html">print</a> <span class="h">&lt;&lt;</span>~<span class="w">EOF</span><span class="sc">;</span></li><li><span class="hh">        This is a here-doc</span></li><li><span class="hh">        EOF</span></li><li><span class="hh">    }</span></li></ol></pre><p>This will print...</p>
<pre class="verbatim"><ol><li>    <span class="w">This</span> <span class="w">is</span> <span class="w">a</span> <span class="w">here</span>-<span class="w">doc</span></li></ol></pre><p>...with no leading whitespace.</p>
<p>The delimiter is used to determine the <b>exact</b> whitespace to
remove from the beginning of each line.  All lines <b>must</b> have
at least the same starting whitespace (except lines only
containing a newline) or perl will croak.  Tabs and spaces can
be mixed, but are matched exactly.  One tab will not be equal to
8 spaces!</p>
<p>Additional beginning whitespace (beyond what preceded the
delimiter) will be preserved:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/print.html">print</a> <span class="h">&lt;&lt;</span>~<span class="w">EOF</span><span class="sc">;</span></li><li><span class="hh">      This text is not indented</span></li><li><span class="hh">        This text is indented with two spaces</span></li><li><span class="hh">      		This text is indented with two tabs</span></li><li><span class="hh">      EOF</span></li></ol></pre><p>Finally, the modifier may be used with all of the forms
mentioned above:</p>
<pre class="verbatim"><ol><li>    <span class="h">&lt;&lt;</span>~\<span class="w">EOF</span><span class="sc">;</span></li><li><span class="hh">    &lt;&lt;~&#39;EOF&#39;</span></li><li><span class="hh">    &lt;&lt;~&quot;EOF&quot;</span></li><li><span class="hh">    &lt;&lt;~`EOF`</span></li></ol></pre><p>And whitespace may be used between the <code class="inline">~</code>
 and quoted delimiters:</p>
<pre class="verbatim"><ol><li>    <span class="h">&lt;&lt;</span>~ <span class="q">&#39;EOF&#39;</span><span class="sc">;</span> <span class="c"># ... &quot;EOF&quot;, `EOF`</span></li></ol></pre></li>
</ul>
<p>It is possible to stack multiple here-docs in a row:</p>
<pre class="verbatim"><ol><li>       print &lt;&lt;"foo", &lt;&lt;"bar"; # you can stack them</li><li>    I said foo.</li><li>    foo</li><li>    I said bar.</li><li>    bar</li><li></li><li>       myfunc(&lt;&lt; "THIS", 23, &lt;&lt;'THAT');</li><li>    Here's a line</li><li>    or two.</li><li>    THIS</li><li>    and here's another.</li><li>    THAT</li></ol></pre><p>Just don't forget that you have to put a semicolon on the end
to finish the statement, as Perl doesn't know you're not going to
try to do this:</p>
<pre class="verbatim"><ol><li>       print &lt;&lt;ABC</li><li>    179231</li><li>    ABC</li><li>       + 20;</li></ol></pre><p>If you want to remove the line terminator from your here-docs,
use <code class="inline"><a class="l_k" href="functions/chomp.html">chomp()</a></code>.</p>
<pre class="verbatim"><ol><li>    chomp($string = &lt;&lt;'END');</li><li>    This is a string.</li><li>    END</li></ol></pre><p>If you want your here-docs to be indented with the rest of the code,
you'll need to remove leading whitespace from each line manually:</p>
<pre class="verbatim"><ol><li>    ($quote = &lt;&lt;'FINIS') =~ s/^\s+//gm;</li><li>       The Road goes ever on and on,</li><li>       down from the door where it began.</li><li>    FINIS</li></ol></pre><p>If you use a here-doc within a delimited construct, such as in <code class="inline"><a class="l_k" href="functions/s.html">s///eg</a></code>,
the quoted material must still come on the line following the
<code class="inline">&lt;&lt;FOO</code> marker, which means it may be inside the delimited
construct:</p>
<pre class="verbatim"><ol><li>    s/this/&lt;&lt;E . 'that'</li><li>    the other</li><li>    E</li><li>     . 'more '/eg;</li></ol></pre><p>It works this way as of Perl 5.18.  Historically, it was inconsistent, and
you would have to write</p>
<pre class="verbatim"><ol><li>    s/this/&lt;&lt;E . 'that'</li><li>     . 'more '/eg;</li><li>    the other</li><li>    E</li></ol></pre><p>outside of string evals.</p>
<p>Additionally, quoting rules for the end-of-string identifier are
unrelated to Perl's quoting rules.  <code class="inline"><a class="l_k" href="functions/q.html">q()</a></code>, <code class="inline"><a class="l_k" href="functions/qq.html">qq()</a></code>, and the like are not
supported in place of <code class="inline"><span class="q">&#39;&#39;</span></code>
 and <code class="inline"><span class="q">&quot;&quot;</span></code>
, and the only interpolation is for
backslashing the quoting character:</p>
<pre class="verbatim"><ol><li>    print &lt;&lt; "abc\"def";</li><li>    testing...</li><li>    abc"def</li></ol></pre><p>Finally, quoted strings cannot span multiple lines.  The general rule is
that the identifier must be a string literal.  Stick with that, and you
should be safe.</p>
</li>
</ul>
<a name="Gory-details-of-parsing-quoted-constructs"></a><h2>Gory details of parsing quoted constructs
</h2>
<p>When presented with something that might have several different
interpretations, Perl uses the <b>DWIM</b> (that's "Do What I Mean")
principle to pick the most probable interpretation.  This strategy
is so successful that Perl programmers often do not suspect the
ambivalence of what they write.  But from time to time, Perl's
notions differ substantially from what the author honestly meant.</p>
<p>This section hopes to clarify how Perl handles quoted constructs.
Although the most common reason to learn this is to unravel labyrinthine
regular expressions, because the initial steps of parsing are the
same for all quoting operators, they are all discussed together.</p>
<p>The most important Perl parsing rule is the first one discussed
below: when processing a quoted construct, Perl first finds the end
of that construct, then interprets its contents.  If you understand
this rule, you may skip the rest of this section on the first
reading.  The other rules are likely to contradict the user's
expectations much less frequently than this first one.</p>
<p>Some passes discussed below are performed concurrently, but because
their results are the same, we consider them individually.  For different
quoting constructs, Perl performs different numbers of passes, from
one to four, but these passes are always performed in the same order.</p>
<ul>
<li><a name="Finding-the-end"></a><b>Finding the end</b>
<p>The first pass is finding the end of the quoted construct.  This results
in saving to a safe location a copy of the text (between the starting
and ending delimiters), normalized as necessary to avoid needing to know
what the original delimiters were.</p>
<p>If the construct is a here-doc, the ending delimiter is a line
that has a terminating string as the content.  Therefore <code class="inline">&lt;&lt;EOF</code> is
terminated by <code class="inline"><span class="w">EOF</span></code>
 immediately followed by <code class="inline"><span class="q">&quot;\n&quot;</span></code>
 and starting
from the first column of the terminating line.
When searching for the terminating line of a here-doc, nothing
is skipped.  In other words, lines after the here-doc syntax
are compared with the terminating string line by line.</p>
<p>For the constructs except here-docs, single characters are used as starting
and ending delimiters.  If the starting delimiter is an opening punctuation
(that is <code class="inline">(</code>, <code class="inline">[</code>, <code class="inline">{</code>, or <code class="inline">&lt;</code>
), the ending delimiter is the
corresponding closing punctuation (that is <code class="inline">)</code>, <code class="inline">]</code>, <code class="inline">}</code>, or <code class="inline">&gt;</code>).
If the starting delimiter is an unpaired character like <code class="inline">/</code> or a closing
punctuation, the ending delimiter is the same as the starting delimiter.
Therefore a <code class="inline">/</code> terminates a <code class="inline"><a class="l_k" href="functions/qq.html">qq//</a></code> construct, while a <code class="inline">]</code> terminates
both <code class="inline"><a class="l_k" href="functions/qq.html">qq[]</a></code> and <code class="inline"><a class="l_k" href="functions/qq.html">qq]]</a></code> constructs.</p>
<p>When searching for single-character delimiters, escaped delimiters
and <code class="inline">\\</code>
 are skipped.  For example, while searching for terminating <code class="inline">/</code>,
combinations of <code class="inline">\\</code>
 and <code class="inline">\/</code> are skipped.  If the delimiters are
bracketing, nested pairs are also skipped.  For example, while searching
for a closing <code class="inline">]</code> paired with the opening <code class="inline">[</code>, combinations of <code class="inline">\\</code>
, <code class="inline">\]</code>,
and <code class="inline">\[</code> are all skipped, and nested <code class="inline">[</code> and <code class="inline">]</code> are skipped as well.
However, when backslashes are used as the delimiters (like <code class="inline"><a class="l_k" href="functions/qq.html">qq\\</a></code> and
<code class="inline"><a class="l_k" href="functions/tr.html">tr\\\</a></code>), nothing is skipped.
During the search for the end, backslashes that escape delimiters or
other backslashes are removed (exactly speaking, they are not copied to the
safe location).</p>
<p>For constructs with three-part delimiters (<code class="inline"><a class="l_k" href="functions/s.html">s///</a></code>, <code class="inline"><a class="l_k" href="functions/y.html">y///</a></code>, and
<code class="inline"><a class="l_k" href="functions/tr.html">tr///</a></code>), the search is repeated once more.
If the first delimiter is not an opening punctuation, the three delimiters must
be the same, such as <code class="inline"><a class="l_k" href="functions/s.html">s!!!</a></code> and <code class="inline"><a class="l_k" href="functions/tr.html">tr)))</a></code>,
in which case the second delimiter
terminates the left part and starts the right part at once.
If the left part is delimited by bracketing punctuation (that is <code class="inline"><span class="s">(</span><span class="s">)</span></code>
,
<code class="inline"><span class="s">[</span><span class="s">]</span></code>
, <code class="inline"><span class="s">{</span><span class="s">}</span></code>
, or <code class="inline">&lt;&gt;</code>
), the right part needs another pair of
delimiters such as <code class="inline"><a class="l_k" href="functions/s.html">s(){}</a></code> and <code class="inline"><a class="l_k" href="functions/tr.html">tr[]//</a></code>.  In these cases, whitespace
and comments are allowed between the two parts, although the comment must follow
at least one whitespace character; otherwise a character expected as the
start of the comment may be regarded as the starting delimiter of the right part.</p>
<p>During this search no attention is paid to the semantics of the construct.
Thus:</p>
<pre class="verbatim"><ol><li>    "$hash{"$foo/$bar"}"</li></ol></pre><p>or:</p>
<pre class="verbatim"><ol><li>    m/</li><li>      bar	# NOT a comment, this slash / terminated m//!</li><li>     /x</li></ol></pre><p>do not form legal quoted expressions.   The quoted part ends on the
first <code class="inline">"</code> and <code class="inline">/</code>, and the rest happens to be a syntax error.
Because the slash that terminated <code class="inline"><a class="l_k" href="functions/m.html">m//</a></code> was followed by a <code class="inline"><span class="w">SPACE</span></code>
,
the example above is not <code class="inline"><a class="l_k" href="functions/m.html">m//x</a></code>, but rather <code class="inline"><a class="l_k" href="functions/m.html">m//</a></code> with no <code class="inline">/x</code>
modifier.  So the embedded <code class="inline"><span class="c">#</span></code>
 is interpreted as a literal <code class="inline"><span class="c">#</span></code>
.</p>
<p>Also no attention is paid to <code class="inline">\<span class="w">c</span>\</code>
 (multichar control char syntax) during
this search.  Thus the second <code class="inline">\</code>
 in <code class="inline"><a class="l_k" href="functions/qq.html">qq/\c\/</a></code> is interpreted as a part
of <code class="inline">\/</code>, and the following <code class="inline">/</code> is not recognized as a delimiter.
Instead, use <code class="inline">\<span class="n">034</span></code>
 or <code class="inline">\<span class="w">x1c</span></code>
 at the end of quoted constructs.</p>
</li>
<li><a name="Interpolation"></a><b>Interpolation
</b>
<p>The next step is interpolation in the text obtained, which is now
delimiter-independent.  There are multiple cases.</p>
<ul>
<li><a name="%3c%3c'EOF'"></a><b><code class="inline">&lt;&lt;'EOF'</code></b>
<p>No interpolation is performed.
Note that the combination <code class="inline">\\</code>
 is left intact, since escaped delimiters
are not available for here-docs.</p>
</li>
<li><a name="m''%2c-the-pattern-of-s'''"></a><b><code class="inline"><a class="l_k" href="functions/m.html">m''</a></code>, the pattern of <code class="inline"><a class="l_k" href="functions/s.html">s'''</a></code></b>
<p>No interpolation is performed at this stage.
Any backslashed sequences including <code class="inline">\\</code>
 are treated at the stage
to <a href="#parsing-regular-expressions">parsing regular expressions</a>.</p>
</li>
<li><a name="''%2c-q%2f%2f%2c-tr'''%2c-y'''%2c-the-replacement-of-s'''"></a><b><code class="inline"><span class="q">&#39;&#39;</span></code>
, <code class="inline"><a class="l_k" href="functions/q.html">q//</a></code>, <code class="inline"><a class="l_k" href="functions/tr.html">tr'''</a></code>, <code class="inline"><a class="l_k" href="functions/y.html">y'''</a></code>, the replacement of <code class="inline"><a class="l_k" href="functions/s.html">s'''</a></code></b>
<p>The only interpolation is removal of <code class="inline">\</code>
 from pairs of <code class="inline">\\</code>
.
Therefore <code class="inline"><span class="q">&quot;-&quot;</span></code>
 in <code class="inline"><a class="l_k" href="functions/tr.html">tr'''</a></code> and <code class="inline"><a class="l_k" href="functions/y.html">y'''</a></code> is treated literally
as a hyphen and no character range is available.
<code class="inline">\<span class="n">1</span></code>
 in the replacement of <code class="inline"><a class="l_k" href="functions/s.html">s'''</a></code> does not work as <code class="inline"><span class="i">$1</span></code>
.</p>
</li>
<li><a name="tr%2f%2f%2f%2c-y%2f%2f%2f"></a><b><code class="inline"><a class="l_k" href="functions/tr.html">tr///</a></code>, <code class="inline"><a class="l_k" href="functions/y.html">y///</a></code></b>
<p>No variable interpolation occurs.  String modifying combinations for
case and quoting such as <code class="inline">\<span class="w">Q</span></code>
, <code class="inline">\<span class="w">U</span></code>
, and <code class="inline">\<span class="w">E</span></code>
 are not recognized.
The other escape sequences such as <code class="inline">\<span class="n">200</span></code>
 and <code class="inline">\<span class="w">t</span></code>
 and backslashed
characters such as <code class="inline">\\</code>
 and <code class="inline">\-</code>
 are converted to appropriate literals.
The character <code class="inline"><span class="q">&quot;-&quot;</span></code>
 is treated specially and therefore <code class="inline">\-</code>
 is treated
as a literal <code class="inline"><span class="q">&quot;-&quot;</span></code>
.</p>
</li>
<li><a name="%22%22%2c-%60%60%2c-qq%2f%2f%2c-qx%2f%2f%2c-%3cfile*glob%3e%2c-%3c%3c%22EOF%22"></a><b><code class="inline"><span class="q">&quot;&quot;</span></code>
, <code class="inline"><span class="q">``</span></code>
, <code class="inline"><a class="l_k" href="functions/qq.html">qq//</a></code>, <code class="inline"><a class="l_k" href="functions/qx.html">qx//</a></code>, <code class="inline"><span class="q">&lt;file*glob&gt;</span></code>
, <code class="inline">&lt;&lt;"EOF"</code></b>
<p><code class="inline">\<span class="w">Q</span></code>
, <code class="inline">\<span class="w">U</span></code>
, <code class="inline">\<span class="w">u</span></code>
, <code class="inline">\<span class="w">L</span></code>
, <code class="inline">\<span class="w">l</span></code>
, <code class="inline">\<span class="w">F</span></code>
 (possibly paired with <code class="inline">\<span class="w">E</span></code>
) are
converted to corresponding Perl constructs.  Thus, <code class="inline"><span class="q">&quot;$foo\Qbaz$bar&quot;</span></code>

is converted to <code class="inline"><span class="i">$foo</span> . <span class="s">(</span><a class="l_k" href="functions/quotemeta.html">quotemeta</a><span class="s">(</span><span class="q">&quot;baz&quot;</span> . <span class="i">$bar</span><span class="s">)</span><span class="s">)</span></code>
 internally.
The other escape sequences such as <code class="inline">\<span class="n">200</span></code>
 and <code class="inline">\<span class="w">t</span></code>
 and backslashed
characters such as <code class="inline">\\</code>
 and <code class="inline">\-</code>
 are replaced with appropriate
expansions.</p>
<p>Let it be stressed that <i>whatever falls between <code class="inline">\<span class="w">Q</span></code>
 and <code class="inline">\<span class="w">E</span></code>
</i>
is interpolated in the usual way.  Something like <code class="inline"><span class="q">&quot;\Q\\E&quot;</span></code>
 has
no <code class="inline">\<span class="w">E</span></code>
 inside.  Instead, it has <code class="inline">\<span class="w">Q</span></code>
, <code class="inline">\\</code>
, and <code class="inline"><span class="w">E</span></code>
, so the
result is the same as for <code class="inline"><span class="q">&quot;\\\\E&quot;</span></code>
.  As a general rule, backslashes
between <code class="inline">\<span class="w">Q</span></code>
 and <code class="inline">\<span class="w">E</span></code>
 may lead to counterintuitive results.  So,
<code class="inline"><span class="q">&quot;\Q\t\E&quot;</span></code>
 is converted to <code class="inline"><a class="l_k" href="functions/quotemeta.html">quotemeta("\t")</a></code>, which is the same
as <code class="inline"><span class="q">&quot;\\\t&quot;</span></code>
 (since TAB is not alphanumeric).  Note also that:</p>
<pre class="verbatim"><ol><li>  <span class="i">$str</span> = <span class="q">&#39;\t&#39;</span><span class="sc">;</span></li><li>  <a class="l_k" href="functions/return.html">return</a> <span class="q">&quot;\Q$str&quot;</span><span class="sc">;</span></li></ol></pre><p>may be closer to the conjectural <i>intention</i> of the writer of <code class="inline"><span class="q">&quot;\Q\t\E&quot;</span></code>
.</p>
<p>Interpolated scalars and arrays are converted internally to the <code class="inline"><a class="l_k" href="functions/join.html">join</a></code> and
<code class="inline"><span class="q">&quot;.&quot;</span></code>
 catenation operations.  Thus, <code class="inline"><span class="q">&quot;$foo XXX &#39;@arr&#39;&quot;</span></code>
 becomes:</p>
<pre class="verbatim"><ol><li>  <span class="i">$foo</span> . <span class="q">&quot; XXX &#39;&quot;</span> . <span class="s">(</span><a class="l_k" href="functions/join.html">join</a> <span class="i">$&quot;</span><span class="cm">,</span> <span class="i">@arr</span><span class="s">)</span> . <span class="q">&quot;&#39;&quot;</span><span class="sc">;</span></li></ol></pre><p>All operations above are performed simultaneously, left to right.</p>
<p>Because the result of <code class="inline">"\Q <i>STRING</i> \E"</code> has all metacharacters
quoted, there is no way to insert a literal <code class="inline"><span class="i">$</span></code>
 or <code class="inline"><span class="i">@</span></code>
 inside a
<code class="inline">\<span class="w">Q</span>\<span class="w">E</span></code>
 pair.  If protected by <code class="inline">\</code>
, <code class="inline"><span class="i">$</span></code>
 will be quoted to become
<code class="inline"><span class="q">&quot;\\\$&quot;</span></code>
; if not, it is interpreted as the start of an interpolated
scalar.</p>
<p>Note also that the interpolation code needs to make a decision on
where the interpolated scalar ends.  For instance, whether
<code class="inline"><span class="q">&quot;a $x -&gt; {c}&quot;</span></code>
 really means:</p>
<pre class="verbatim"><ol><li>  <span class="q">&quot;a &quot;</span> . <span class="i">$x</span> . <span class="q">&quot; -&gt; {c}&quot;</span><span class="sc">;</span></li></ol></pre><p>or:</p>
<pre class="verbatim"><ol><li>  <span class="q">&quot;a &quot;</span> . <span class="i">$x</span> -&gt; {<span class="w">c</span>}<span class="sc">;</span></li></ol></pre><p>Most of the time, the longest possible text that does not include
spaces between components and which contains matching braces or
brackets.  because the outcome may be determined by voting based
on heuristic estimators, the result is not strictly predictable.
Fortunately, it's usually correct for ambiguous cases.</p>
</li>
<li><a name="the-replacement-of-s%2f%2f%2f"></a><b>the replacement of <code class="inline"><a class="l_k" href="functions/s.html">s///</a></code></b>
<p>Processing of <code class="inline">\<span class="w">Q</span></code>
, <code class="inline">\<span class="w">U</span></code>
, <code class="inline">\<span class="w">u</span></code>
, <code class="inline">\<span class="w">L</span></code>
, <code class="inline">\<span class="w">l</span></code>
, <code class="inline">\<span class="w">F</span></code>
 and interpolation
happens as with <code class="inline"><a class="l_k" href="functions/qq.html">qq//</a></code> constructs.</p>
<p>It is at this step that <code class="inline">\<span class="n">1</span></code>
 is begrudgingly converted to <code class="inline"><span class="i">$1</span></code>
 in
the replacement text of <code class="inline"><a class="l_k" href="functions/s.html">s///</a></code>, in order to correct the incorrigible
<i>sed</i> hackers who haven't picked up the saner idiom yet.  A warning
is emitted if the <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">warnings</span></code>
 pragma or the <b>-w</b> command-line flag
(that is, the <code class="inline"><span class="i">$^W</span></code>
 variable) was set.</p>
</li>
<li><a name="RE-in-m%3fRE%3f%2c-%2fRE%2f%2c-m%2fRE%2f%2c-s%2fRE%2ffoo%2f%2c"></a><b><code class="inline"><span class="w">RE</span></code>
 in <code class="inline"><a class="l_k" href="functions/m.html">m?RE?</a></code>, <code class="inline"><span class="q">/RE/</span></code>
, <code class="inline"><a class="l_k" href="functions/m.html">m/RE/</a></code>, <code class="inline"><a class="l_k" href="functions/s.html">s/RE/foo/</a></code>,</b>
<p>Processing of <code class="inline">\<span class="w">Q</span></code>
, <code class="inline">\<span class="w">U</span></code>
, <code class="inline">\<span class="w">u</span></code>
, <code class="inline">\<span class="w">L</span></code>
, <code class="inline">\<span class="w">l</span></code>
, <code class="inline">\<span class="w">F</span></code>
, <code class="inline">\<span class="w">E</span></code>
,
and interpolation happens (almost) as with <code class="inline"><a class="l_k" href="functions/qq.html">qq//</a></code> constructs.</p>
<p>Processing of <code class="inline">\<span class="i">N</span><span class="s">{</span>...<span class="s">}</span></code>
 is also done here, and compiled into an intermediate
form for the regex compiler.  (This is because, as mentioned below, the regex
compilation may be done at execution time, and <code class="inline">\<span class="i">N</span><span class="s">{</span>...<span class="s">}</span></code>
 is a compile-time
construct.)</p>
<p>However any other combinations of <code class="inline">\</code>
 followed by a character
are not substituted but only skipped, in order to parse them
as regular expressions at the following step.
As <code class="inline">\<span class="w">c</span></code>
 is skipped at this step, <code class="inline"><span class="i">@</span></code>
 of <code class="inline">\<span class="w">c</span><span class="i">@</span></code>
 in RE is possibly
treated as an array symbol (for example <code class="inline"><span class="i">@foo</span></code>
),
even though the same text in <code class="inline"><a class="l_k" href="functions/qq.html">qq//</a></code> gives interpolation of <code class="inline">\<span class="w">c</span><span class="i">@</span></code>
.</p>
<p>Code blocks such as <code class="inline">(?{BLOCK})</code> are handled by temporarily passing control
back to the perl parser, in a similar way that an interpolated array
subscript expression such as <code class="inline"><span class="q">&quot;foo$array[1+f(&quot;</span><span class="s">[</span><span class="w">xyz</span><span class="q">&quot;)]bar&quot;</span></code>
 would be.</p>
<p>Moreover, inside <code class="inline">(?{BLOCK})</code>, <code class="inline">(?# comment )</code>, and
a <code class="inline"><span class="c">#</span></code>
-comment in a <code class="inline">/x</code>-regular expression, no processing is
performed whatsoever.  This is the first step at which the presence
of the <code class="inline">/x</code> modifier is relevant.</p>
<p>Interpolation in patterns has several quirks: <code class="inline"><span class="i">$|</span></code>
, <code class="inline"><span class="i">$(</span></code>
, <code class="inline"><span class="i">$)</span></code>
, <code class="inline"><span class="i">@+</span></code>

and <code class="inline"><span class="i">@-</span></code>
 are not interpolated, and constructs <code class="inline"><span class="i">$var</span>[<span class="w">SOMETHING</span>]</code>
 are
voted (by several different estimators) to be either an array element
or <code class="inline"><span class="i">$var</span></code>
 followed by an RE alternative.  This is where the notation
<code class="inline"><span class="i">$</span>{<span class="w">arr</span><span class="s">[</span><span class="i">$bar</span><span class="s">]</span>}</code>
 comes handy: <code class="inline"><span class="q">/${arr[0-9]}/</span></code>
 is interpreted as
array element <code class="inline"><span class="n">-9</span></code>
, not as a regular expression from the variable
<code class="inline"><span class="i">$arr</span></code>
 followed by a digit, which would be the interpretation of
<code class="inline"><span class="q">/$arr[0-9]/</span></code>
.  Since voting among different estimators may occur,
the result is not predictable.</p>
<p>The lack of processing of <code class="inline">\\</code>
 creates specific restrictions on
the post-processed text.  If the delimiter is <code class="inline">/</code>, one cannot get
the combination <code class="inline">\/</code> into the result of this step.  <code class="inline">/</code> will
finish the regular expression, <code class="inline">\/</code> will be stripped to <code class="inline">/</code> on
the previous step, and <code class="inline">\\/</code> will be left as is.  Because <code class="inline">/</code> is
equivalent to <code class="inline">\/</code> inside a regular expression, this does not
matter unless the delimiter happens to be character special to the
RE engine, such as in <code class="inline"><a class="l_k" href="functions/s.html">s*foo*bar*</a></code>, <code class="inline"><a class="l_k" href="functions/m.html">m[foo]</a></code>, or <code class="inline"><a class="l_k" href="functions/m.html">m?foo?</a></code>; or an
alphanumeric char, as in:</p>
<pre class="verbatim"><ol><li>  <span class="q">m m ^ a \s* b mmx</span><span class="sc">;</span></li></ol></pre><p>In the RE above, which is intentionally obfuscated for illustration, the
delimiter is <code class="inline"><a class="l_k" href="functions/m.html">m</a></code>, the modifier is <code class="inline"><span class="w">mx</span></code>
, and after delimiter-removal the
RE is the same as for <code class="inline"><span class="q">m/ ^ a \s* b /mx</span></code>
.  There's more than one
reason you're encouraged to restrict your delimiters to non-alphanumeric,
non-whitespace choices.</p>
</li>
</ul>
<p>This step is the last one for all constructs except regular expressions,
which are processed further.</p>
</li>
<li><a name="parsing-regular-expressions"></a><b>parsing regular expressions
</b>
<p>Previous steps were performed during the compilation of Perl code,
but this one happens at run time, although it may be optimized to
be calculated at compile time if appropriate.  After preprocessing
described above, and possibly after evaluation if concatenation,
joining, casing translation, or metaquoting are involved, the
resulting <i>string</i> is passed to the RE engine for compilation.</p>
<p>Whatever happens in the RE engine might be better discussed in <a href="perlre.html">perlre</a>,
but for the sake of continuity, we shall do so here.</p>
<p>This is another step where the presence of the <code class="inline">/x</code> modifier is
relevant.  The RE engine scans the string from left to right and
converts it into a finite automaton.</p>
<p>Backslashed characters are either replaced with corresponding
literal strings (as with <code class="inline">\{</code>), or else they generate special nodes
in the finite automaton (as with <code class="inline">\<span class="w">b</span></code>
).  Characters special to the
RE engine (such as <code class="inline">|</code>) generate corresponding nodes or groups of
nodes.  <code class="inline">(?#...)</code> comments are ignored.  All the rest is either
converted to literal strings to match, or else is ignored (as is
whitespace and <code class="inline"><span class="c">#</span></code>
-style comments if <code class="inline">/x</code> is present).</p>
<p>Parsing of the bracketed character class construct, <code class="inline"><span class="s">[</span>...<span class="s">]</span></code>
, is
rather different than the rule used for the rest of the pattern.
The terminator of this construct is found using the same rules as
for finding the terminator of a <code class="inline"><span class="s">{</span><span class="s">}</span></code>
-delimited construct, the only
exception being that <code class="inline">]</code> immediately following <code class="inline">[</code> is treated as
though preceded by a backslash.</p>
<p>The terminator of runtime <code class="inline">(?{...})</code> is found by temporarily switching
control to the perl parser, which should stop at the point where the
logically balancing terminating <code class="inline">}</code> is found.</p>
<p>It is possible to inspect both the string given to RE engine and the
resulting finite automaton.  See the arguments <code class="inline"><span class="w">debug</span></code>
/<code class="inline"><span class="w">debugcolor</span></code>

in the <code class="inline">use <a href="re.html">re</a></code> pragma, as well as Perl's <b>-Dr</b> command-line
switch documented in <a href="perlrun.html#Command-Switches">Command Switches in perlrun</a>.</p>
</li>
<li><a name="Optimization-of-regular-expressions"></a><b>Optimization of regular expressions
</b>
<p>This step is listed for completeness only.  Since it does not change
semantics, details of this step are not documented and are subject
to change without notice.  This step is performed over the finite
automaton that was generated during the previous pass.</p>
<p>It is at this stage that <code class="inline"><a class="l_k" href="functions/split.html">split()</a></code> silently optimizes <code class="inline"><span class="q">/^/</span></code>
 to
mean <code class="inline"><span class="q">/^/m</span></code>
.</p>
</li>
</ul>
<a name="I%2fO-Operators"></a><h2>I/O Operators
    
  </h2>
<p>There are several I/O operators you should know about.</p>
<p>A string enclosed by backticks (grave accents) first undergoes
double-quote interpolation.  It is then interpreted as an external
command, and the output of that command is the value of the
backtick string, like in a shell.  In scalar context, a single string
consisting of all output is returned.  In list context, a list of
values is returned, one per line of output.  (You can set <code class="inline"><span class="i">$/</span></code>
 to use
a different line terminator.)  The command is executed each time the
pseudo-literal is evaluated.  The status value of the command is
returned in <code class="inline"><span class="i">$?</span></code>
 (see <a href="perlvar.html">perlvar</a> for the interpretation of <code class="inline"><span class="i">$?</span></code>
).
Unlike in <b>csh</b>, no translation is done on the return data--newlines
remain newlines.  Unlike in any of the shells, single quotes do not
hide variable names in the command from interpretation.  To pass a
literal dollar-sign through to the shell you need to hide it with a
backslash.  The generalized form of backticks is <code class="inline"><a class="l_k" href="functions/qx.html">qx//</a></code>.  (Because
backticks always undergo shell expansion as well, see <a href="perlsec.html">perlsec</a> for
security concerns.)
    </p>
<p>In scalar context, evaluating a filehandle in angle brackets yields
the next line from that file (the newline, if any, included), or
<code class="inline"><a class="l_k" href="functions/undef.html">undef</a></code> at end-of-file or on error.  When <code class="inline"><span class="i">$/</span></code>
 is set to <code class="inline"><a class="l_k" href="functions/undef.html">undef</a></code>
(sometimes known as file-slurp mode) and the file is empty, it
returns <code class="inline"><span class="q">&#39;&#39;</span></code>
 the first time, followed by <code class="inline"><a class="l_k" href="functions/undef.html">undef</a></code> subsequently.</p>
<p>Ordinarily you must assign the returned value to a variable, but
there is one situation where an automatic assignment happens.  If
and only if the input symbol is the only thing inside the conditional
of a <code class="inline">while</code>
 statement (even if disguised as a <code class="inline">for<span class="s">(</span><span class="sc">;</span><span class="sc">;</span><span class="s">)</span></code>
 loop),
the value is automatically assigned to the global variable <code class="inline"><span class="i">$_</span></code>
,
destroying whatever was there previously.  (This may seem like an
odd thing to you, but you'll use the construct in almost every Perl
script you write.)  The <code class="inline"><span class="i">$_</span></code>
 variable is not implicitly localized.
You'll have to put a <code class="inline"><a class="l_k" href="functions/local.html">local</a> <span class="i">$_</span><span class="sc">;</span></code>
 before the loop if you want that
to happen.</p>
<p>The following lines are equivalent:</p>
<pre class="verbatim"><ol><li>    while <span class="s">(</span><a class="l_k" href="functions/defined.html">defined</a><span class="s">(</span><span class="i">$_</span> = <span class="q">&lt;STDIN&gt;</span><span class="s">)</span><span class="s">)</span> <span class="s">{</span> <a class="l_k" href="functions/print.html">print</a><span class="sc">;</span> <span class="s">}</span></li><li>    while <span class="s">(</span><span class="i">$_</span> = <span class="q">&lt;STDIN&gt;</span><span class="s">)</span> <span class="s">{</span> <a class="l_k" href="functions/print.html">print</a><span class="sc">;</span> <span class="s">}</span></li><li>    while <span class="s">(</span><span class="q">&lt;STDIN&gt;</span><span class="s">)</span> <span class="s">{</span> <a class="l_k" href="functions/print.html">print</a><span class="sc">;</span> <span class="s">}</span></li><li>    for <span class="s">(</span><span class="sc">;</span><span class="q">&lt;STDIN&gt;</span><span class="sc">;</span><span class="s">)</span> <span class="s">{</span> <a class="l_k" href="functions/print.html">print</a><span class="sc">;</span> <span class="s">}</span></li><li>    <a class="l_k" href="functions/print.html">print</a> while <a class="l_k" href="functions/defined.html">defined</a><span class="s">(</span><span class="i">$_</span> = <span class="q">&lt;STDIN&gt;</span><span class="s">)</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/print.html">print</a> while <span class="s">(</span><span class="i">$_</span> = <span class="q">&lt;STDIN&gt;</span><span class="s">)</span><span class="sc">;</span></li><li>    <a class="l_k" href="functions/print.html">print</a> while <span class="q">&lt;STDIN&gt;</span><span class="sc">;</span></li></ol></pre><p>This also behaves similarly, but assigns to a lexical variable
instead of to <code class="inline"><span class="i">$_</span></code>
:</p>
<pre class="verbatim"><ol><li>    while <span class="s">(</span><a class="l_k" href="functions/my.html">my</a> <span class="i">$line</span> = <span class="q">&lt;STDIN&gt;</span><span class="s">)</span> <span class="s">{</span> <a class="l_k" href="functions/print.html">print</a> <span class="i">$line</span> <span class="s">}</span></li></ol></pre><p>In these loop constructs, the assigned value (whether assignment
is automatic or explicit) is then tested to see whether it is
defined.  The defined test avoids problems where the line has a string
value that would be treated as false by Perl; for example a "" or
a <code class="inline"><span class="q">&quot;0&quot;</span></code>
 with no trailing newline.  If you really mean for such values
to terminate the loop, they should be tested for explicitly:</p>
<pre class="verbatim"><ol><li>    while <span class="s">(</span><span class="s">(</span><span class="i">$_</span> = <span class="q">&lt;STDIN&gt;</span><span class="s">)</span> ne <span class="q">&#39;0&#39;</span><span class="s">)</span> <span class="s">{</span> ... <span class="s">}</span></li><li>    while <span class="s">(</span><span class="q">&lt;STDIN&gt;</span><span class="s">)</span> <span class="s">{</span> <a class="l_k" href="functions/last.html">last</a> unless <span class="i">$_</span><span class="sc">;</span> ... <span class="s">}</span></li></ol></pre><p>In other boolean contexts, <code class="inline">&lt;<i>FILEHANDLE</i>&gt;</code> without an
explicit <code class="inline"><a class="l_k" href="functions/defined.html">defined</a></code> test or comparison elicits a warning if the
<code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">warnings</span></code>
 pragma or the <b>-w</b>
command-line switch (the <code class="inline"><span class="i">$^W</span></code>
 variable) is in effect.</p>
<p>The filehandles STDIN, STDOUT, and STDERR are predefined.  (The
filehandles <code class="inline"><span class="w">stdin</span></code>
, <code class="inline"><span class="w">stdout</span></code>
, and <code class="inline"><span class="w">stderr</span></code>
 will also work except
in packages, where they would be interpreted as local identifiers
rather than global.)  Additional filehandles may be created with
the <code class="inline"><a class="l_k" href="functions/open.html">open()</a></code> function, amongst others.  See <a href="perlopentut.html">perlopentut</a> and
<a href="functions/open.html">open</a> for details on this.
  </p>
<p>If a <code class="inline">&lt;<i>FILEHANDLE</i>&gt;</code> is used in a context that is looking for
a list, a list comprising all input lines is returned, one line per
list element.  It's easy to grow to a rather large data space this
way, so use with care.</p>
<p><code class="inline">&lt;<i>FILEHANDLE</i>&gt;</code>  may also be spelled <code class="inline">readline(*<i>FILEHANDLE</i>)</code>.
See <a href="functions/readline.html">readline</a>.</p>
<p>The null filehandle <code class="inline">&lt;&gt;</code>
 is special: it can be used to emulate the
behavior of <b>sed</b> and <b>awk</b>, and any other Unix filter program
that takes a list of filenames, doing the same to each line
of input from all of them.  Input from <code class="inline">&lt;&gt;</code>
 comes either from
standard input, or from each file listed on the command line.  Here's
how it works: the first time <code class="inline">&lt;&gt;</code>
 is evaluated, the <code class="inline"><span class="i">@ARGV</span></code>
 array is
checked, and if it is empty, <code class="inline"><span class="i">$ARGV</span>[<span class="n">0</span>]</code>
 is set to <code class="inline"><span class="q">&quot;-&quot;</span></code>
, which when opened
gives you standard input.  The <code class="inline"><span class="i">@ARGV</span></code>
 array is then processed as a list
of filenames.  The loop</p>
<pre class="verbatim"><ol><li>    while <span class="s">(</span>&lt;&gt;<span class="s">)</span> <span class="s">{</span></li><li>	...			<span class="c"># code for each line</span></li><li>    <span class="s">}</span></li></ol></pre><p>is equivalent to the following Perl-like pseudo code:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/unshift.html">unshift</a><span class="s">(</span><span class="i">@ARGV</span><span class="cm">,</span> <span class="q">&#39;-&#39;</span><span class="s">)</span> unless <span class="i">@ARGV</span><span class="sc">;</span></li><li>    while <span class="s">(</span><span class="i">$ARGV</span> = <a class="l_k" href="functions/shift.html">shift</a><span class="s">)</span> <span class="s">{</span></li><li>	<a class="l_k" href="functions/open.html">open</a><span class="s">(</span><span class="w">ARGV</span><span class="cm">,</span> <span class="i">$ARGV</span><span class="s">)</span><span class="sc">;</span></li><li>	while <span class="s">(</span><span class="q">&lt;ARGV&gt;</span><span class="s">)</span> <span class="s">{</span></li><li>	    ...		<span class="c"># code for each line</span></li><li>	<span class="s">}</span></li><li>    <span class="s">}</span></li></ol></pre><p>except that it isn't so cumbersome to say, and will actually work.
It really does shift the <code class="inline"><span class="i">@ARGV</span></code>
 array and put the current filename
into the <code class="inline"><span class="i">$ARGV</span></code>
 variable.  It also uses filehandle <i>ARGV</i>
internally.  <code class="inline">&lt;&gt;</code>
 is just a synonym for <code class="inline"><span class="q">&lt;ARGV&gt;</span></code>
, which
is magical.  (The pseudo code above doesn't work because it treats
<code class="inline"><span class="q">&lt;ARGV&gt;</span></code>
 as non-magical.)</p>
<p>Since the null filehandle uses the two argument form of <a href="functions/open.html">open</a>
it interprets special characters, so if you have a script like this:</p>
<pre class="verbatim"><ol><li>    while <span class="s">(</span>&lt;&gt;<span class="s">)</span> <span class="s">{</span></li><li>        <a class="l_k" href="functions/print.html">print</a><span class="sc">;</span></li><li>    <span class="s">}</span></li></ol></pre><p>and call it with <code class="inline"><span class="w">perl</span> <span class="w">dangerous</span>.<span class="w">pl</span> <span class="q">'rm -rfv *|'</span></code>
, it actually opens a
pipe, executes the <code class="inline"><span class="w">rm</span></code>
 command and reads <code class="inline"><span class="w">rm</span></code>
's output from that pipe.
If you want all items in <code class="inline"><span class="i">@ARGV</span></code>
 to be interpreted as file names, you
can use the module <code class="inline"><span class="w">ARGV::readonly</span></code>
 from CPAN, or use the double bracket:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/while.html">while</a> <span class="s">(</span><span class="h">&lt;&lt;</span>&gt;&gt;<span class="s">)</span> <span class="s">{</span></li><li><span class="hh">        print;</span></li><li><span class="hh">    }</span></li></ol></pre><p>Using double angle brackets inside of a while causes the open to use the
three argument form (with the second argument being <code class="inline">&lt;</code>
), so all
arguments in <code class="inline"><span class="w">ARGV</span></code>
 are treated as literal filenames (including <code class="inline"><span class="q">&quot;-&quot;</span></code>
).
(Note that for convenience, if you use <code class="inline"><span class="h">&lt;&lt;</span>&gt;&gt;</code>
 and if <code class="inline"><span class="i">@ARGV</span></code>
 is
empty, it will still read from the standard input.)</p>
<p>You can modify <code class="inline"><span class="i">@ARGV</span></code>
 before the first <code class="inline">&lt;&gt;</code>
 as long as the array ends up
containing the list of filenames you really want.  Line numbers (<code class="inline"><span class="i">$.</span></code>
)
continue as though the input were one big happy file.  See the example
in <a href="functions/eof.html">eof</a> for how to reset line numbers on each file.</p>
<p>If you want to set <code class="inline"><span class="i">@ARGV</span></code>
 to your own list of files, go right ahead.
This sets <code class="inline"><span class="i">@ARGV</span></code>
 to all plain text files if no <code class="inline"><span class="i">@ARGV</span></code>
 was given:</p>
<pre class="verbatim"><ol><li>    <span class="i">@ARGV</span> = <a class="l_k" href="functions/grep.html">grep</a> <span class="s">{</span> -f &amp;&amp; -T <span class="s">}</span> <a class="l_k" href="functions/glob.html">glob</a><span class="s">(</span><span class="q">&#39;*&#39;</span><span class="s">)</span> unless <span class="i">@ARGV</span><span class="sc">;</span></li></ol></pre><p>You can even set them to pipe commands.  For example, this automatically
filters compressed arguments through <b>gzip</b>:</p>
<pre class="verbatim"><ol><li>    <span class="i">@ARGV</span> = <a class="l_k" href="functions/map.html">map</a> <span class="s">{</span> <span class="q">/\.(gz|Z)$/</span> ? <span class="q">&quot;gzip -dc &lt; $_ |&quot;</span> <span class="co">:</span> <span class="i">$_</span> <span class="s">}</span> <span class="i">@ARGV</span><span class="sc">;</span></li></ol></pre><p>If you want to pass switches into your script, you can use one of the
<code class="inline"><span class="w">Getopts</span></code>
 modules or put a loop on the front like this:</p>
<pre class="verbatim"><ol><li>    while <span class="s">(</span><span class="i">$_</span> = <span class="i">$ARGV</span>[<span class="n">0</span>]<span class="cm">,</span> <span class="q">/^-/</span><span class="s">)</span> <span class="s">{</span></li><li>	<a class="l_k" href="functions/shift.html">shift</a><span class="sc">;</span></li><li>        <a class="l_k" href="functions/last.html">last</a> if <span class="q">/^--$/</span><span class="sc">;</span></li><li>	if <span class="s">(</span><span class="q">/^-D(.*)/</span><span class="s">)</span> <span class="s">{</span> <span class="i">$debug</span> = <span class="i">$1</span> <span class="s">}</span></li><li>	if <span class="s">(</span><span class="q">/^-v/</span><span class="s">)</span>     <span class="s">{</span> <span class="i">$verbose</span>++  <span class="s">}</span></li><li>	<span class="c"># ...		# other switches</span></li><li>    <span class="s">}</span></li><li></li><li>    while <span class="s">(</span>&lt;&gt;<span class="s">)</span> <span class="s">{</span></li><li>	<span class="c"># ...		# code for each line</span></li><li>    <span class="s">}</span></li></ol></pre><p>The <code class="inline">&lt;&gt;</code>
 symbol will return <code class="inline"><a class="l_k" href="functions/undef.html">undef</a></code> for end-of-file only once.
If you call it again after this, it will assume you are processing another
<code class="inline"><span class="i">@ARGV</span></code>
 list, and if you haven't set <code class="inline"><span class="i">@ARGV</span></code>
, will read input from STDIN.</p>
<p>If what the angle brackets contain is a simple scalar variable (for example,
<code class="inline"><span class="i">$foo</span></code>
), then that variable contains the name of the
filehandle to input from, or its typeglob, or a reference to the
same.  For example:</p>
<pre class="verbatim"><ol><li>    <span class="i">$fh</span> = \<span class="i">*STDIN</span><span class="sc">;</span></li><li>    <span class="i">$line</span> = <span class="q">&lt;$fh&gt;</span><span class="sc">;</span></li></ol></pre><p>If what's within the angle brackets is neither a filehandle nor a simple
scalar variable containing a filehandle name, typeglob, or typeglob
reference, it is interpreted as a filename pattern to be globbed, and
either a list of filenames or the next filename in the list is returned,
depending on context.  This distinction is determined on syntactic
grounds alone.  That means <code class="inline"><span class="q">&lt;$x&gt;</span></code>
 is always a <code class="inline"><a class="l_k" href="functions/readline.html">readline()</a></code> from
an indirect handle, but <code class="inline"><span class="q">&lt;$hash{key}&gt;</span></code>
 is always a <code class="inline"><a class="l_k" href="functions/glob.html">glob()</a></code>.
That's because <code class="inline"><span class="i">$x</span></code>
 is a simple scalar variable, but <code class="inline"><span class="i">$hash</span>{<span class="w">key</span>}</code>
 is
not--it's a hash element.  Even <code class="inline"><span class="q">&lt;$x &gt;</span></code>
 (note the extra space)
is treated as <code class="inline"><a class="l_k" href="functions/glob.html">glob</a><span class="s">(</span><span class="q">&quot;$x &quot;</span><span class="s">)</span></code>
, not <code class="inline"><a class="l_k" href="functions/readline.html">readline($x)</a></code>.</p>
<p>One level of double-quote interpretation is done first, but you can't
say <code class="inline"><span class="q">&lt;$foo&gt;</span></code>
 because that's an indirect filehandle as explained
in the previous paragraph.  (In older versions of Perl, programmers
would insert curly brackets to force interpretation as a filename glob:
<code class="inline"><span class="q">&lt;${foo}&gt;</span></code>
.  These days, it's considered cleaner to call the
internal function directly as <code class="inline"><a class="l_k" href="functions/glob.html">glob($foo)</a></code>, which is probably the right
way to have done it in the first place.)  For example:</p>
<pre class="verbatim"><ol><li>    while <span class="s">(</span><span class="q">&lt;*.c&gt;</span><span class="s">)</span> <span class="s">{</span></li><li>	<a class="l_k" href="functions/chmod.html">chmod</a> <span class="n">0644</span><span class="cm">,</span> <span class="i">$_</span><span class="sc">;</span></li><li>    <span class="s">}</span></li></ol></pre><p>is roughly equivalent to:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/open.html">open</a><span class="s">(</span><span class="w">FOO</span><span class="cm">,</span> <span class="q">&quot;echo *.c | tr -s &#39; \t\r\f&#39; &#39;\\012\\012\\012\\012&#39;|&quot;</span><span class="s">)</span><span class="sc">;</span></li><li>    while <span class="s">(</span><span class="q">&lt;FOO&gt;</span><span class="s">)</span> <span class="s">{</span></li><li>	<a class="l_k" href="functions/chomp.html">chomp</a><span class="sc">;</span></li><li>	<a class="l_k" href="functions/chmod.html">chmod</a> <span class="n">0644</span><span class="cm">,</span> <span class="i">$_</span><span class="sc">;</span></li><li>    <span class="s">}</span></li></ol></pre><p>except that the globbing is actually done internally using the standard
<code class="inline"><a href="File/Glob.html">File::Glob</a></code> extension.  Of course, the shortest way to do the above is:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/chmod.html">chmod</a> <span class="n">0644</span><span class="cm">,</span> <span class="q">&lt;*.c&gt;</span><span class="sc">;</span></li></ol></pre><p>A (file)glob evaluates its (embedded) argument only when it is
starting a new list.  All values must be read before it will start
over.  In list context, this isn't important because you automatically
get them all anyway.  However, in scalar context the operator returns
the next value each time it's called, or <code class="inline"><a class="l_k" href="functions/undef.html">undef</a></code> when the list has
run out.  As with filehandle reads, an automatic <code class="inline"><a class="l_k" href="functions/defined.html">defined</a></code> is
generated when the glob occurs in the test part of a <code class="inline">while</code>
,
because legal glob returns (for example,
a file called <i>0</i>) would otherwise
terminate the loop.  Again, <code class="inline"><a class="l_k" href="functions/undef.html">undef</a></code> is returned only once.  So if
you're expecting a single value from a glob, it is much better to
say</p>
<pre class="verbatim"><ol><li>    <span class="s">(</span><span class="i">$file</span><span class="s">)</span> = <span class="q">&lt;blurch*&gt;</span><span class="sc">;</span></li></ol></pre><p>than</p>
<pre class="verbatim"><ol><li>    <span class="i">$file</span> = <span class="q">&lt;blurch*&gt;</span><span class="sc">;</span></li></ol></pre><p>because the latter will alternate between returning a filename and
returning false.</p>
<p>If you're trying to do variable interpolation, it's definitely better
to use the <code class="inline"><a class="l_k" href="functions/glob.html">glob()</a></code> function, because the older notation can cause people
to become confused with the indirect filehandle notation.</p>
<pre class="verbatim"><ol><li>    <span class="i">@files</span> = <a class="l_k" href="functions/glob.html">glob</a><span class="s">(</span><span class="q">&quot;$dir/*.[ch]&quot;</span><span class="s">)</span><span class="sc">;</span></li><li>    <span class="i">@files</span> = <a class="l_k" href="functions/glob.html">glob</a><span class="s">(</span><span class="i">$files</span>[<span class="i">$i</span>]<span class="s">)</span><span class="sc">;</span></li></ol></pre><a name="Constant-Folding"></a><h2>Constant Folding
 </h2>
<p>Like C, Perl does a certain amount of expression evaluation at
compile time whenever it determines that all arguments to an
operator are static and have no side effects.  In particular, string
concatenation happens at compile time between literals that don't do
variable substitution.  Backslash interpolation also happens at
compile time.  You can say</p>
<pre class="verbatim"><ol><li>      <span class="q">&#39;Now is the time for all&#39;</span></li><li>    . <span class="q">&quot;\n&quot;</span></li><li>    .  <span class="q">&#39;good men to come to.&#39;</span></li></ol></pre><p>and this all reduces to one string internally.  Likewise, if
you say</p>
<pre class="verbatim"><ol><li>    foreach <span class="i">$file</span> <span class="s">(</span><span class="i">@filenames</span><span class="s">)</span> <span class="s">{</span></li><li>	if <span class="s">(</span>-s <span class="i">$file</span> &gt; <span class="n">5</span> + <span class="n">100</span> * <span class="n">2</span>**<span class="n">16</span><span class="s">)</span> <span class="s">{</span>  <span class="s">}</span></li><li>    <span class="s">}</span></li></ol></pre><p>the compiler precomputes the number which that expression
represents so that the interpreter won't have to.</p>
<a name="No-ops"></a><h2>No-ops
 </h2>
<p>Perl doesn't officially have a no-op operator, but the bare constants
<code class="inline"><span class="n">0</span></code>
 and <code class="inline"><span class="n">1</span></code>
 are special-cased not to produce a warning in void
context, so you can for example safely do</p>
<pre class="verbatim"><ol><li>    <span class="n">1</span> while <span class="i">foo</span><span class="s">(</span><span class="s">)</span><span class="sc">;</span></li></ol></pre><a name="Bitwise-String-Operators"></a><h2>Bitwise String Operators
    </h2>
<p>Bitstrings of any size may be manipulated by the bitwise operators
(<code class="inline">~ | &amp; ^</code>).</p>
<p>If the operands to a binary bitwise op are strings of different
sizes, <b>|</b> and <b>^</b> ops act as though the shorter operand had
additional zero bits on the right, while the <b>&amp;</b> op acts as though
the longer operand were truncated to the length of the shorter.
The granularity for such extension or truncation is one or more
bytes.</p>
<pre class="verbatim"><ol><li>    <span class="c"># ASCII-based examples</span></li><li>    <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;j p \n&quot;</span> ^ <span class="q">&quot; a h&quot;</span><span class="sc">;</span>        	<span class="c"># prints &quot;JAPH\n&quot;</span></li><li>    <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;JA&quot;</span> | <span class="q">&quot;  ph\n&quot;</span><span class="sc">;</span>          	<span class="c"># prints &quot;japh\n&quot;</span></li><li>    <a class="l_k" href="functions/print.html">print</a> <span class="q">&quot;japh\nJunk&quot;</span> &amp; <span class="q">'_____'</span><span class="sc">;</span>   	<span class="c"># prints &quot;JAPH\n&quot;;</span></li><li>    <a class="l_k" href="functions/print.html">print</a> <span class="q">'p N$'</span> ^ <span class="q">&quot; E&lt;H\n&quot;</span><span class="sc">;</span>		<span class="c"># prints &quot;Perl\n&quot;;</span></li></ol></pre><p>If you are intending to manipulate bitstrings, be certain that
you're supplying bitstrings: If an operand is a number, that will imply
a <b>numeric</b> bitwise operation.  You may explicitly show which type of
operation you intend by using <code class="inline"><span class="q">&quot;&quot;</span></code>
 or <code class="inline"><span class="n">0</span>+</code>
, as in the examples below.</p>
<pre class="verbatim"><ol><li>    <span class="i">$foo</span> =  <span class="n">150</span>  |  <span class="n">105</span><span class="sc">;</span>	<span class="c"># yields 255  (0x96 | 0x69 is 0xFF)</span></li><li>    <span class="i">$foo</span> = <span class="q">&#39;150&#39;</span> |  <span class="n">105</span><span class="sc">;</span>	<span class="c"># yields 255</span></li><li>    <span class="i">$foo</span> =  <span class="n">150</span>  | <span class="q">&#39;105&#39;</span><span class="sc">;</span>	<span class="c"># yields 255</span></li><li>    <span class="i">$foo</span> = <span class="q">&#39;150&#39;</span> | <span class="q">&#39;105&#39;</span><span class="sc">;</span>	<span class="c"># yields string &#39;155&#39; (under ASCII)</span></li><li></li><li>    <span class="i">$baz</span> = <span class="n">0</span>+<span class="i">$foo</span> &amp; <span class="n">0</span>+<span class="i">$bar</span><span class="sc">;</span>	<span class="c"># both ops explicitly numeric</span></li><li>    <span class="i">$biz</span> = <span class="q">&quot;$foo&quot;</span> ^ <span class="q">&quot;$bar&quot;</span><span class="sc">;</span>	<span class="c"># both ops explicitly stringy</span></li></ol></pre><p>This somewhat unpredictable behavior can be avoided with the experimental
"bitwise" feature, new in Perl 5.22.  You can enable it via <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">feature</span>
<span class="q">&#39;bitwise&#39;</span></code>
.  By default, it will warn unless the <code class="inline"><span class="q">&quot;experimental::bitwise&quot;</span></code>

warnings category has been disabled.  (<code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">experimental</span> <span class="q">&#39;bitwise&#39;</span></code>
 will
enable the feature and disable the warning.)  Under this feature, the four
standard bitwise operators (<code class="inline">~ | &amp; ^</code>) are always numeric.  Adding a dot
after each operator (<code class="inline">~. |. <span class="i">&amp;.</span> ^.</code>
) forces it to treat its operands as
strings:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/use.html">use</a> <span class="w">experimental</span> <span class="q">&quot;bitwise&quot;</span><span class="sc">;</span></li><li>    <span class="i">$foo</span> =  <span class="n">150</span>  |  <span class="n">105</span><span class="sc">;</span>	<span class="c"># yields 255  (0x96 | 0x69 is 0xFF)</span></li><li>    <span class="i">$foo</span> = <span class="q">&#39;150&#39;</span> |  <span class="n">105</span><span class="sc">;</span>	<span class="c"># yields 255</span></li><li>    <span class="i">$foo</span> =  <span class="n">150</span>  | <span class="q">&#39;105&#39;</span><span class="sc">;</span>	<span class="c"># yields 255</span></li><li>    <span class="i">$foo</span> = <span class="q">&#39;150&#39;</span> | <span class="q">&#39;105&#39;</span><span class="sc">;</span>	<span class="c"># yields 255</span></li><li>    <span class="i">$foo</span> =  <span class="n">150</span>  |. <span class="n">105</span><span class="sc">;</span>	<span class="c"># yields string &#39;155&#39;</span></li><li>    <span class="i">$foo</span> = <span class="q">&#39;150&#39;</span> |. <span class="n">105</span><span class="sc">;</span>	<span class="c"># yields string &#39;155&#39;</span></li><li>    <span class="i">$foo</span> =  <span class="n">150</span>  |.<span class="q">&#39;105&#39;</span><span class="sc">;</span>	<span class="c"># yields string &#39;155&#39;</span></li><li>    <span class="i">$foo</span> = <span class="q">&#39;150&#39;</span> |.<span class="q">&#39;105&#39;</span><span class="sc">;</span>	<span class="c"># yields string &#39;155&#39;</span></li><li></li><li>    <span class="i">$baz</span> = <span class="i">$foo</span> &amp;  <span class="i">$bar</span><span class="sc">;</span>	<span class="c"># both operands numeric</span></li><li>    <span class="i">$biz</span> = <span class="i">$foo</span> ^. <span class="i">$bar</span><span class="sc">;</span>	<span class="c"># both operands stringy</span></li></ol></pre><p>The assignment variants of these operators (<code class="inline">&amp;= |= ^= <span class="i">&amp;.</span>= |.= ^.=</code>
)
behave likewise under the feature.</p>
<p>The behavior of these operators is problematic (and subject to change)
if either or both of the strings are encoded in UTF-8 (see
<a href="perlunicode.html#Byte-and-Character-Semantics">Byte and Character Semantics in perlunicode</a>.</p>
<p>See <a href="functions/vec.html">vec</a> for information on how to manipulate individual bits
in a bit vector.</p>
<a name="Integer-Arithmetic"></a><h2>Integer Arithmetic
</h2>
<p>By default, Perl assumes that it must do most of its arithmetic in
floating point.  But by saying</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/use.html">use</a> <span class="w">integer</span><span class="sc">;</span></li></ol></pre><p>you may tell the compiler to use integer operations
(see <a href="integer.html">integer</a> for a detailed explanation) from here to the end of
the enclosing BLOCK.  An inner BLOCK may countermand this by saying</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/no.html">no</a> <span class="w">integer</span><span class="sc">;</span></li></ol></pre><p>which lasts until the end of that BLOCK.  Note that this doesn't
mean everything is an integer, merely that Perl will use integer
operations for arithmetic, comparison, and bitwise operators.  For
example, even under <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">integer</span></code>
, if you take the <code class="inline"><a class="l_k" href="functions/sqrt.html">sqrt(2)</a></code>, you'll
still get <code class="inline"><span class="n">1.4142135623731</span></code>
 or so.</p>
<p>Used on numbers, the bitwise operators (<code class="inline"><span class="i">&amp;</span></code>
 <code class="inline">|</code> <code class="inline">^</code> <code class="inline">~</code>
 <code class="inline">&lt;&lt;</code>

<code class="inline">&gt;&gt;</code>
) always produce integral results.  (But see also
<a href="#Bitwise-String-Operators">Bitwise String Operators</a>.)  However, <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">integer</span></code>
 still has meaning for
them.  By default, their results are interpreted as unsigned integers, but
if <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">integer</span></code>
 is in effect, their results are interpreted
as signed integers.  For example, <code class="inline">~<span class="n">0</span></code>
 usually evaluates to a large
integral value.  However, <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">integer</span><span class="sc">;</span> ~<span class="n">0</span></code>
 is <code class="inline"><span class="n">-1</span></code>
 on two's-complement
machines.</p>
<a name="Floating-point-Arithmetic"></a><h2>Floating-point Arithmetic</h2>
<p>   </p>
<p>While <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">integer</span></code>
 provides integer-only arithmetic, there is no
analogous mechanism to provide automatic rounding or truncation to a
certain number of decimal places.  For rounding to a certain number
of digits, <code class="inline"><a class="l_k" href="functions/sprintf.html">sprintf()</a></code> or <code class="inline"><a class="l_k" href="functions/printf.html">printf()</a></code> is usually the easiest route.
See <a href="perlfaq4.html">perlfaq4</a>.</p>
<p>Floating-point numbers are only approximations to what a mathematician
would call real numbers.  There are infinitely more reals than floats,
so some corners must be cut.  For example:</p>
<pre class="verbatim"><ol><li>    <a class="l_k" href="functions/printf.html">printf</a> <span class="q">&quot;%.20g\n&quot;</span><span class="cm">,</span> <span class="n">123456789123456789</span><span class="sc">;</span></li><li>    <span class="c">#        produces 123456789123456784</span></li></ol></pre><p>Testing for exact floating-point equality or inequality is not a
good idea.  Here's a (relatively expensive) work-around to compare
whether two floating-point numbers are equal to a particular number of
decimal places.  See Knuth, volume II, for a more robust treatment of
this topic.</p>
<pre class="verbatim"><ol><li><a name="fp_equal"></a>    sub <span class="m">fp_equal</span> <span class="s">{</span></li><li>	<a class="l_k" href="functions/my.html">my</a> <span class="s">(</span><span class="i">$X</span><span class="cm">,</span> <span class="i">$Y</span><span class="cm">,</span> <span class="i">$POINTS</span><span class="s">)</span> = <span class="i">@_</span><span class="sc">;</span></li><li>	<a class="l_k" href="functions/my.html">my</a> <span class="s">(</span><span class="i">$tX</span><span class="cm">,</span> <span class="i">$tY</span><span class="s">)</span><span class="sc">;</span></li><li>	<span class="i">$tX</span> = <a class="l_k" href="functions/sprintf.html">sprintf</a><span class="s">(</span><span class="q">&quot;%.${POINTS}g&quot;</span><span class="cm">,</span> <span class="i">$X</span><span class="s">)</span><span class="sc">;</span></li><li>	<span class="i">$tY</span> = <a class="l_k" href="functions/sprintf.html">sprintf</a><span class="s">(</span><span class="q">&quot;%.${POINTS}g&quot;</span><span class="cm">,</span> <span class="i">$Y</span><span class="s">)</span><span class="sc">;</span></li><li>	<a class="l_k" href="functions/return.html">return</a> <span class="i">$tX</span> eq <span class="i">$tY</span><span class="sc">;</span></li><li>    <span class="s">}</span></li></ol></pre><p>The POSIX module (part of the standard perl distribution) implements
<code class="inline"><span class="i">ceil</span><span class="s">(</span><span class="s">)</span></code>
, <code class="inline"><span class="i">floor</span><span class="s">(</span><span class="s">)</span></code>
, and other mathematical and trigonometric functions.
The <code class="inline"><a href="Math/Complex.html">Math::Complex</a></code> module (part of the standard perl distribution)
defines mathematical functions that work on both the reals and the
imaginary numbers.  <code class="inline"><span class="w">Math::Complex</span></code>
 is not as efficient as POSIX, but
POSIX can't work with complex numbers.</p>
<p>Rounding in financial applications can have serious implications, and
the rounding method used should be specified precisely.  In these
cases, it probably pays not to trust whichever system rounding is
being used by Perl, but to instead implement the rounding function you
need yourself.</p>
<a name="Bigger-Numbers"></a><h2>Bigger Numbers
</h2>
<p>The standard <code class="inline"><a href="Math/BigInt.html">Math::BigInt</a></code>, <code class="inline"><a href="Math/BigRat.html">Math::BigRat</a></code>, and
<code class="inline"><a href="Math/BigFloat.html">Math::BigFloat</a></code> modules,
along with the <code class="inline"><span class="w">bignum</span></code>
, <code class="inline"><span class="w">bigint</span></code>
, and <code class="inline"><span class="w">bigrat</span></code>
 pragmas, provide
variable-precision arithmetic and overloaded operators, although
they're currently pretty slow.  At the cost of some space and
considerable speed, they avoid the normal pitfalls associated with
limited-precision representations.</p>
<pre class="verbatim"><ol><li>	<a class="l_k" href="functions/use.html">use</a> <span class="n">5.010</span><span class="sc">;</span></li><li>	<a class="l_k" href="functions/use.html">use</a> <span class="w">bigint</span><span class="sc">;</span>  <span class="c"># easy interface to Math::BigInt</span></li><li>	<span class="i">$x</span> = <span class="n">123456789123456789</span><span class="sc">;</span></li><li>	<a class="l_k" href="functions/say.html">say</a> <span class="i">$x</span> * <span class="i">$x</span><span class="sc">;</span></li><li>    <span class="n">+15241578780673678515622620750190521</span></li></ol></pre><p>Or with rationals:</p>
<pre class="verbatim"><ol><li>        <a class="l_k" href="functions/use.html">use</a> <span class="n">5.010</span><span class="sc">;</span></li><li>        <a class="l_k" href="functions/use.html">use</a> <span class="w">bigrat</span><span class="sc">;</span></li><li>        <span class="i">$x</span> = <span class="n">3</span>/<span class="n">22</span><span class="sc">;</span></li><li>        <span class="i">$y</span> = <span class="n">4</span>/<span class="n">6</span><span class="sc">;</span></li><li>        <a class="l_k" href="functions/say.html">say</a> <span class="q">&quot;x/y is &quot;</span><span class="cm">,</span> <span class="i">$x</span>/<span class="i">$y</span><span class="sc">;</span></li><li>        <a class="l_k" href="functions/say.html">say</a> <span class="q">&quot;x*y is &quot;</span><span class="cm">,</span> <span class="i">$x</span>*<span class="i">$y</span><span class="sc">;</span></li><li>        <span class="w">x</span>/<span class="q">y is 9/44</span></li><li>        <span class="q">        x*y is 1/11</span></li></ol></pre><p>Several modules let you calculate with unlimited or fixed precision
(bound only by memory and CPU time).  There
are also some non-standard modules that
provide faster implementations via external C libraries.</p>
<p>Here is a short, but incomplete summary:</p>
<pre class="verbatim"><ol><li>  <span class="w">Math::String</span>           <span class="w">treat</span> <span class="w">string</span> <span class="w">sequences</span> <span class="w">like</span> <span class="w">numbers</span></li><li>  <span class="w">Math::FixedPrecision</span>   <span class="w">calculate</span> <span class="w">with</span> <span class="w">a</span> <span class="w">fixed</span> <span class="w">precision</span></li><li>  <span class="w">Math::Currency</span>         <a class="l_k" href="functions/for.html">for</a> <span class="w">currency</span> <span class="w">calculations</span></li><li>  <span class="w">Bit::Vector</span>            <span class="w">manipulate</span> <span class="w">bit</span> <span class="w">vectors</span> <span class="w">fast</span> <span class="s">(</span><span class="w">uses</span> <span class="w">C</span><span class="s">)</span></li><li>  <span class="w">Math::BigIntFast</span>       <span class="w">Bit::Vector</span> <span class="w">wrapper</span> <a class="l_k" href="functions/for.html">for</a> <span class="w">big</span> <span class="w">numbers</span></li><li>  <span class="w">Math::Pari</span>             <span class="w">provides</span> <span class="w">access</span> <span class="w">to</span> <span class="w">the</span> <span class="w">Pari</span> <span class="w">C</span> <span class="w">library</span></li><li>  <span class="w">Math::Cephes</span>           <span class="w">uses</span> <span class="w">the</span> <span class="w">external</span> <span class="w">Cephes</span> <span class="w">C</span> <span class="w">library</span> <span class="s">(</span><a class="l_k" href="functions/no.html">no</a></li><li>                         <span class="w">big</span> <span class="w">numbers</span><span class="s">)</span></li><li>  <span class="w">Math::Cephes::Fraction</span> <span class="w">fractions</span> <span class="w">via</span> <span class="w">the</span> <span class="w">Cephes</span> <span class="w">library</span></li><li>  <span class="w">Math::GMP</span>              <span class="w">another</span> <span class="w">one</span> <span class="w">using</span> <span class="w">an</span> <span class="w">external</span> <span class="w">C</span> <span class="w">library</span></li><li>  <span class="w">Math::GMPz</span>             <span class="w">an</span> <span class="w">alternative</span> <span class="w">interface</span> <span class="w">to</span> <span class="w">libgmp&#39;s</span> <span class="w">big</span> <span class="w">ints</span></li><li>  <span class="w">Math::GMPq</span>             <span class="w">an</span> <span class="w">interface</span> <span class="w">to</span> <span class="w">libgmp&#39;s</span> <span class="w">fraction</span> <span class="w">numbers</span></li><li>  <span class="w">Math::GMPf</span>             <span class="w">an</span> <span class="w">interface</span> <span class="w">to</span> <span class="w">libgmp&#39;s</span> <span class="w">floating</span> <span class="w">point</span> <span class="w">numbers</span></li></ol></pre><p>Choose wisely.</p>




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</a><li><a href="#DESCRIPTION">DESCRIPTION</a><ul><li><a href="#Operator-Precedence-and-Associativity">Operator Precedence and Associativity
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 </a><li><a href="#Binding-Operators">Binding Operators
   </a><li><a href="#Multiplicative-Operators">Multiplicative Operators
</a><li><a href="#Additive-Operators">Additive Operators
</a><li><a href="#Shift-Operators">Shift Operators
  
   
   </a><li><a href="#Named-Unary-Operators">Named Unary Operators
</a><li><a href="#Relational-Operators">Relational Operators
 </a><li><a href="#Equality-Operators">Equality Operators
   </a><li><a href="#Smartmatch-Operator">Smartmatch Operator</a><li><a href="#Bitwise-And">Bitwise And
  </a><li><a href="#Bitwise-Or-and-Exclusive-Or">Bitwise Or and Exclusive Or
   
 </a><li><a href="#C-style-Logical-And">C-style Logical And
  </a><li><a href="#C-style-Logical-Or">C-style Logical Or
 </a><li><a href="#Logical-Defined-Or">Logical Defined-Or
 </a><li><a href="#Range-Operators">Range Operators
   </a><li><a href="#Conditional-Operator">Conditional Operator
   </a><li><a href="#Assignment-Operators">Assignment Operators
      
        
     </a><li><a href="#Comma-Operator">Comma Operator
  </a><li><a href="#List-Operators-(Rightward)">List Operators (Rightward)
 </a><li><a href="#Logical-Not">Logical Not
 </a><li><a href="#Logical-And">Logical And
 </a><li><a href="#Logical-or-and-Exclusive-Or">Logical or and Exclusive Or
 

 </a><li><a href="#C-Operators-Missing-From-Perl">C Operators Missing From Perl
  
 </a><li><a href="#Quote-and-Quote-like-Operators">Quote and Quote-like Operators
      
          
 </a><li><a href="#Regexp-Quote-Like-Operators">Regexp Quote-Like Operators
</a><li><a href="#Quote-Like-Operators">Quote-Like Operators
</a><li><a href="#Gory-details-of-parsing-quoted-constructs">Gory details of parsing quoted constructs
</a><li><a href="#I%2fO-Operators">I/O Operators
    
  </a><li><a href="#Constant-Folding">Constant Folding
 </a><li><a href="#No-ops">No-ops
 </a><li><a href="#Bitwise-String-Operators">Bitwise String Operators
    </a><li><a href="#Integer-Arithmetic">Integer Arithmetic
</a><li><a href="#Floating-point-Arithmetic">Floating-point Arithmetic</a><li><a href="#Bigger-Numbers">Bigger Numbers
</a></ul></ul>
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