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<h1>substr</h1>
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<h1>substr</h1>
<div class="mod_az_list">
<a href="../index-functions.html">Perl functions A-Z</a> |
<a href="../index-functions-by-cat.html">Perl functions by category</a> |
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<ul>
<li><a name="substr-EXPR%2cOFFSET%2cLENGTH%2cREPLACEMENT"></a><b>substr EXPR,OFFSET,LENGTH,REPLACEMENT</b>
<p> </p>
</li>
</ul>
<ul>
<li><a name="substr-EXPR%2cOFFSET%2cLENGTH"></a><b>substr EXPR,OFFSET,LENGTH</b>
</li>
<li><a name="substr-EXPR%2cOFFSET"></a><b>substr EXPR,OFFSET</b>
<p>Extracts a substring out of EXPR and returns it. First character is at
offset <code class="inline"><span class="n">0</span></code>
, or whatever you've set <code class="inline"><span class="i">$[</span></code>
to (but don't do that).
If OFFSET is negative (or more precisely, less than <code class="inline"><span class="i">$[</span></code>
), starts
that far from the end of the string. If LENGTH is omitted, returns
everything to the end of the string. If LENGTH is negative, leaves that
many characters off the end of the string.</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="../functions/my.html">my</a> <span class="i">$s</span> = <span class="q">"The black cat climbed the green tree"</span><span class="sc">;</span></li><li> <a class="l_k" href="../functions/my.html">my</a> <span class="i">$color</span> = <a class="l_k" href="../functions/substr.html">substr</a> <span class="i">$s</span><span class="cm">,</span> <span class="n">4</span><span class="cm">,</span> <span class="n">5</span><span class="sc">;</span> <span class="c"># black</span></li><li> <a class="l_k" href="../functions/my.html">my</a> <span class="i">$middle</span> = <a class="l_k" href="../functions/substr.html">substr</a> <span class="i">$s</span><span class="cm">,</span> <span class="n">4</span><span class="cm">,</span> <span class="n">-11</span><span class="sc">;</span> <span class="c"># black cat climbed the</span></li><li> <a class="l_k" href="../functions/my.html">my</a> <span class="i">$end</span> = <a class="l_k" href="../functions/substr.html">substr</a> <span class="i">$s</span><span class="cm">,</span> <span class="n">14</span><span class="sc">;</span> <span class="c"># climbed the green tree</span></li><li> <a class="l_k" href="../functions/my.html">my</a> <span class="i">$tail</span> = <a class="l_k" href="../functions/substr.html">substr</a> <span class="i">$s</span><span class="cm">,</span> <span class="n">-4</span><span class="sc">;</span> <span class="c"># tree</span></li><li> <a class="l_k" href="../functions/my.html">my</a> <span class="i">$z</span> = <a class="l_k" href="../functions/substr.html">substr</a> <span class="i">$s</span><span class="cm">,</span> <span class="n">-4</span><span class="cm">,</span> <span class="n">2</span><span class="sc">;</span> <span class="c"># tr</span></li></ol></pre><p>You can use the substr() function as an lvalue, in which case EXPR
must itself be an lvalue. If you assign something shorter than LENGTH,
the string will shrink, and if you assign something longer than LENGTH,
the string will grow to accommodate it. To keep the string the same
length you may need to pad or chop your value using <code class="inline"><a class="l_k" href="../functions/sprintf.html">sprintf</a></code>.</p>
<p>If OFFSET and LENGTH specify a substring that is partly outside the
string, only the part within the string is returned. If the substring
is beyond either end of the string, substr() returns the undefined
value and produces a warning. When used as an lvalue, specifying a
substring that is entirely outside the string is a fatal error.
Here's an example showing the behavior for boundary cases:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="../functions/my.html">my</a> <span class="i">$name</span> = <span class="q">'fred'</span><span class="sc">;</span></li><li> <a class="l_k" href="../functions/substr.html">substr</a><span class="s">(</span><span class="i">$name</span><span class="cm">,</span> <span class="n">4</span><span class="s">)</span> = <span class="q">'dy'</span><span class="sc">;</span> <span class="c"># $name is now 'freddy'</span></li><li> <a class="l_k" href="../functions/my.html">my</a> <span class="i">$null</span> = <a class="l_k" href="../functions/substr.html">substr</a> <span class="i">$name</span><span class="cm">,</span> <span class="n">6</span><span class="cm">,</span> <span class="n">2</span><span class="sc">;</span> <span class="c"># returns '' (no warning)</span></li><li> <a class="l_k" href="../functions/my.html">my</a> <span class="i">$oops</span> = <a class="l_k" href="../functions/substr.html">substr</a> <span class="i">$name</span><span class="cm">,</span> <span class="n">7</span><span class="sc">;</span> <span class="c"># returns undef, with warning</span></li><li> <a class="l_k" href="../functions/substr.html">substr</a><span class="s">(</span><span class="i">$name</span><span class="cm">,</span> <span class="n">7</span><span class="s">)</span> = <span class="q">'gap'</span><span class="sc">;</span> <span class="c"># fatal error</span></li></ol></pre><p>An alternative to using substr() as an lvalue is to specify the
replacement string as the 4th argument. This allows you to replace
parts of the EXPR and return what was there before in one operation,
just as you can with splice().</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="../functions/my.html">my</a> <span class="i">$s</span> = <span class="q">"The black cat climbed the green tree"</span><span class="sc">;</span></li><li> <a class="l_k" href="../functions/my.html">my</a> <span class="i">$z</span> = <a class="l_k" href="../functions/substr.html">substr</a> <span class="i">$s</span><span class="cm">,</span> <span class="n">14</span><span class="cm">,</span> <span class="n">7</span><span class="cm">,</span> <span class="q">"jumped from"</span><span class="sc">;</span> <span class="c"># climbed</span></li><li> <span class="c"># $s is now "The black cat jumped from the green tree"</span></li></ol></pre><p>Note that the lvalue returned by the 3-arg version of substr() acts as
a 'magic bullet'; each time it is assigned to, it remembers which part
of the original string is being modified; for example:</p>
<pre class="verbatim"><ol><li> <span class="i">$x</span> = <span class="q">'1234'</span><span class="sc">;</span></li><li> for <span class="s">(</span><a class="l_k" href="../functions/substr.html">substr</a><span class="s">(</span><span class="i">$x</span><span class="cm">,</span><span class="n">1</span><span class="cm">,</span><span class="n">2</span><span class="s">)</span><span class="s">)</span> <span class="s">{</span></li><li> <span class="i">$_</span> = <span class="q">'a'</span><span class="sc">;</span> <a class="l_k" href="../functions/print.html">print</a> <span class="i">$x</span><span class="cm">,</span><span class="q">"\n"</span><span class="sc">;</span> <span class="c"># prints 1a4</span></li><li> <span class="i">$_</span> = <span class="q">'xyz'</span><span class="sc">;</span> <a class="l_k" href="../functions/print.html">print</a> <span class="i">$x</span><span class="cm">,</span><span class="q">"\n"</span><span class="sc">;</span> <span class="c"># prints 1xyz4</span></li><li> <span class="i">$x</span> = <span class="q">'56789'</span><span class="sc">;</span></li><li> <span class="i">$_</span> = <span class="q">'pq'</span><span class="sc">;</span> <a class="l_k" href="../functions/print.html">print</a> <span class="i">$x</span><span class="cm">,</span><span class="q">"\n"</span><span class="sc">;</span> <span class="c"># prints 5pq9</span></li><li> <span class="s">}</span></li></ol></pre><p>Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
unspecified.</p>
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