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<h1 class="title toc-ignore">Regular expressions</h1>



<p>Regular expressions are a concise and flexible tool for describing
patterns in strings. This vignette describes the key features of
stringr’s regular expressions, as implemented by <a href="https://github.com/gagolews/stringi">stringi</a>. It is not a
tutorial, so if you’re unfamiliar regular expressions, I’d recommend
starting at <a href="https://r4ds.had.co.nz/strings.html" class="uri">https://r4ds.had.co.nz/strings.html</a>. If you want to
master the details, I’d recommend reading the classic <a href="https://www.amazon.com/Mastering-Regular-Expressions-Jeffrey-Friedl/dp/0596528124"><em>Mastering
Regular Expressions</em></a> by Jeffrey E. F. Friedl.</p>
<p>Regular expressions are the default pattern engine in stringr. That
means when you use a pattern matching function with a bare string, it’s
equivalent to wrapping it in a call to <code>regex()</code>:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb1-1"><a href="#cb1-1" tabindex="-1"></a><span class="co"># The regular call:</span></span>
<span id="cb1-2"><a href="#cb1-2" tabindex="-1"></a><span class="fu">str_extract</span>(fruit, <span class="st">&quot;nana&quot;</span>)</span>
<span id="cb1-3"><a href="#cb1-3" tabindex="-1"></a><span class="co"># Is shorthand for</span></span>
<span id="cb1-4"><a href="#cb1-4" tabindex="-1"></a><span class="fu">str_extract</span>(fruit, <span class="fu">regex</span>(<span class="st">&quot;nana&quot;</span>))</span></code></pre></div>
<p>You will need to use <code>regex()</code> explicitly if you want to
override the default options, as you’ll see in examples below.</p>
<div id="basic-matches" class="section level2">
<h2>Basic matches</h2>
<p>The simplest patterns match exact strings:</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb2-1"><a href="#cb2-1" tabindex="-1"></a>x <span class="ot">&lt;-</span> <span class="fu">c</span>(<span class="st">&quot;apple&quot;</span>, <span class="st">&quot;banana&quot;</span>, <span class="st">&quot;pear&quot;</span>)</span>
<span id="cb2-2"><a href="#cb2-2" tabindex="-1"></a><span class="fu">str_extract</span>(x, <span class="st">&quot;an&quot;</span>)</span>
<span id="cb2-3"><a href="#cb2-3" tabindex="-1"></a><span class="co">#&gt; [1] NA   &quot;an&quot; NA</span></span></code></pre></div>
<p>You can perform a case-insensitive match using
<code>ignore_case = TRUE</code>:</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb3-1"><a href="#cb3-1" tabindex="-1"></a>bananas <span class="ot">&lt;-</span> <span class="fu">c</span>(<span class="st">&quot;banana&quot;</span>, <span class="st">&quot;Banana&quot;</span>, <span class="st">&quot;BANANA&quot;</span>)</span>
<span id="cb3-2"><a href="#cb3-2" tabindex="-1"></a><span class="fu">str_detect</span>(bananas, <span class="st">&quot;banana&quot;</span>)</span>
<span id="cb3-3"><a href="#cb3-3" tabindex="-1"></a><span class="co">#&gt; [1]  TRUE FALSE FALSE</span></span>
<span id="cb3-4"><a href="#cb3-4" tabindex="-1"></a><span class="fu">str_detect</span>(bananas, <span class="fu">regex</span>(<span class="st">&quot;banana&quot;</span>, <span class="at">ignore_case =</span> <span class="cn">TRUE</span>))</span>
<span id="cb3-5"><a href="#cb3-5" tabindex="-1"></a><span class="co">#&gt; [1] TRUE TRUE TRUE</span></span></code></pre></div>
<p>The next step up in complexity is <code>.</code>, which matches any
character except a newline:</p>
<div class="sourceCode" id="cb4"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb4-1"><a href="#cb4-1" tabindex="-1"></a><span class="fu">str_extract</span>(x, <span class="st">&quot;.a.&quot;</span>)</span>
<span id="cb4-2"><a href="#cb4-2" tabindex="-1"></a><span class="co">#&gt; [1] NA    &quot;ban&quot; &quot;ear&quot;</span></span></code></pre></div>
<p>You can allow <code>.</code> to match everything, including
<code>\n</code>, by setting <code>dotall = TRUE</code>:</p>
<div class="sourceCode" id="cb5"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb5-1"><a href="#cb5-1" tabindex="-1"></a><span class="fu">str_detect</span>(<span class="st">&quot;</span><span class="sc">\n</span><span class="st">X</span><span class="sc">\n</span><span class="st">&quot;</span>, <span class="st">&quot;.X.&quot;</span>)</span>
<span id="cb5-2"><a href="#cb5-2" tabindex="-1"></a><span class="co">#&gt; [1] FALSE</span></span>
<span id="cb5-3"><a href="#cb5-3" tabindex="-1"></a><span class="fu">str_detect</span>(<span class="st">&quot;</span><span class="sc">\n</span><span class="st">X</span><span class="sc">\n</span><span class="st">&quot;</span>, <span class="fu">regex</span>(<span class="st">&quot;.X.&quot;</span>, <span class="at">dotall =</span> <span class="cn">TRUE</span>))</span>
<span id="cb5-4"><a href="#cb5-4" tabindex="-1"></a><span class="co">#&gt; [1] TRUE</span></span></code></pre></div>
</div>
<div id="escaping" class="section level2">
<h2>Escaping</h2>
<p>If “<code>.</code>” matches any character, how do you match a literal
“<code>.</code>”? You need to use an “escape” to tell the regular
expression you want to match it exactly, not use its special behaviour.
Like strings, regexps use the backslash, <code>\</code>, to escape
special behaviour. So to match an <code>.</code>, you need the regexp
<code>\.</code>. Unfortunately this creates a problem. We use strings to
represent regular expressions, and <code>\</code> is also used as an
escape symbol in strings. So to create the regular expression
<code>\.</code> we need the string <code>&quot;\\.&quot;</code>.</p>
<div class="sourceCode" id="cb6"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb6-1"><a href="#cb6-1" tabindex="-1"></a><span class="co"># To create the regular expression, we need \\</span></span>
<span id="cb6-2"><a href="#cb6-2" tabindex="-1"></a>dot <span class="ot">&lt;-</span> <span class="st">&quot;</span><span class="sc">\\</span><span class="st">.&quot;</span></span>
<span id="cb6-3"><a href="#cb6-3" tabindex="-1"></a></span>
<span id="cb6-4"><a href="#cb6-4" tabindex="-1"></a><span class="co"># But the expression itself only contains one:</span></span>
<span id="cb6-5"><a href="#cb6-5" tabindex="-1"></a><span class="fu">writeLines</span>(dot)</span>
<span id="cb6-6"><a href="#cb6-6" tabindex="-1"></a><span class="co">#&gt; \.</span></span>
<span id="cb6-7"><a href="#cb6-7" tabindex="-1"></a></span>
<span id="cb6-8"><a href="#cb6-8" tabindex="-1"></a><span class="co"># And this tells R to look for an explicit .</span></span>
<span id="cb6-9"><a href="#cb6-9" tabindex="-1"></a><span class="fu">str_extract</span>(<span class="fu">c</span>(<span class="st">&quot;abc&quot;</span>, <span class="st">&quot;a.c&quot;</span>, <span class="st">&quot;bef&quot;</span>), <span class="st">&quot;a</span><span class="sc">\\</span><span class="st">.c&quot;</span>)</span>
<span id="cb6-10"><a href="#cb6-10" tabindex="-1"></a><span class="co">#&gt; [1] NA    &quot;a.c&quot; NA</span></span></code></pre></div>
<p>If <code>\</code> is used as an escape character in regular
expressions, how do you match a literal <code>\</code>? Well you need to
escape it, creating the regular expression <code>\\</code>. To create
that regular expression, you need to use a string, which also needs to
escape <code>\</code>. That means to match a literal <code>\</code> you
need to write <code>&quot;\\\\&quot;</code> — you need four backslashes to match
one!</p>
<div class="sourceCode" id="cb7"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb7-1"><a href="#cb7-1" tabindex="-1"></a>x <span class="ot">&lt;-</span> <span class="st">&quot;a</span><span class="sc">\\</span><span class="st">b&quot;</span></span>
<span id="cb7-2"><a href="#cb7-2" tabindex="-1"></a><span class="fu">writeLines</span>(x)</span>
<span id="cb7-3"><a href="#cb7-3" tabindex="-1"></a><span class="co">#&gt; a\b</span></span>
<span id="cb7-4"><a href="#cb7-4" tabindex="-1"></a></span>
<span id="cb7-5"><a href="#cb7-5" tabindex="-1"></a><span class="fu">str_extract</span>(x, <span class="st">&quot;</span><span class="sc">\\\\</span><span class="st">&quot;</span>)</span>
<span id="cb7-6"><a href="#cb7-6" tabindex="-1"></a><span class="co">#&gt; [1] &quot;\\&quot;</span></span></code></pre></div>
<p>In this vignette, I use <code>\.</code> to denote the regular
expression, and <code>&quot;\\.&quot;</code> to denote the string that represents
the regular expression.</p>
<p>An alternative quoting mechanism is <code>\Q...\E</code>: all the
characters in <code>...</code> are treated as exact matches. This is
useful if you want to exactly match user input as part of a regular
expression.</p>
<div class="sourceCode" id="cb8"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb8-1"><a href="#cb8-1" tabindex="-1"></a>x <span class="ot">&lt;-</span> <span class="fu">c</span>(<span class="st">&quot;a.b.c.d&quot;</span>, <span class="st">&quot;aeb&quot;</span>)</span>
<span id="cb8-2"><a href="#cb8-2" tabindex="-1"></a>starts_with <span class="ot">&lt;-</span> <span class="st">&quot;a.b&quot;</span></span>
<span id="cb8-3"><a href="#cb8-3" tabindex="-1"></a></span>
<span id="cb8-4"><a href="#cb8-4" tabindex="-1"></a><span class="fu">str_detect</span>(x, <span class="fu">paste0</span>(<span class="st">&quot;^&quot;</span>, starts_with))</span>
<span id="cb8-5"><a href="#cb8-5" tabindex="-1"></a><span class="co">#&gt; [1] TRUE TRUE</span></span>
<span id="cb8-6"><a href="#cb8-6" tabindex="-1"></a><span class="fu">str_detect</span>(x, <span class="fu">paste0</span>(<span class="st">&quot;^</span><span class="sc">\\</span><span class="st">Q&quot;</span>, starts_with, <span class="st">&quot;</span><span class="sc">\\</span><span class="st">E&quot;</span>))</span>
<span id="cb8-7"><a href="#cb8-7" tabindex="-1"></a><span class="co">#&gt; [1]  TRUE FALSE</span></span></code></pre></div>
</div>
<div id="special-characters" class="section level2">
<h2>Special characters</h2>
<p>Escapes also allow you to specify individual characters that are
otherwise hard to type. You can specify individual unicode characters in
five ways, either as a variable number of hex digits (four is most
common), or by name:</p>
<ul>
<li><p><code>\xhh</code>: 2 hex digits.</p></li>
<li><p><code>\x{hhhh}</code>: 1-6 hex digits.</p></li>
<li><p><code>\uhhhh</code>: 4 hex digits.</p></li>
<li><p><code>\Uhhhhhhhh</code>: 8 hex digits.</p></li>
<li><p><code>\N{name}</code>, e.g. <code>\N{grinning face}</code>
matches the basic smiling emoji.</p></li>
</ul>
<p>Similarly, you can specify many common control characters:</p>
<ul>
<li><p><code>\a</code>: bell.</p></li>
<li><p><code>\cX</code>: match a control-X character.</p></li>
<li><p><code>\e</code>: escape (<code>\u001B</code>).</p></li>
<li><p><code>\f</code>: form feed (<code>\u000C</code>).</p></li>
<li><p><code>\n</code>: line feed (<code>\u000A</code>).</p></li>
<li><p><code>\r</code>: carriage return (<code>\u000D</code>).</p></li>
<li><p><code>\t</code>: horizontal tabulation
(<code>\u0009</code>).</p></li>
<li><p><code>\0ooo</code> match an octal character. ‘ooo’ is from one to
three octal digits, from 000 to 0377. The leading zero is
required.</p></li>
</ul>
<p>(Many of these are only of historical interest and are only included
here for the sake of completeness.)</p>
</div>
<div id="matching-multiple-characters" class="section level2">
<h2>Matching multiple characters</h2>
<p>There are a number of patterns that match more than one character.
You’ve already seen <code>.</code>, which matches any character (except
a newline). A closely related operator is <code>\X</code>, which matches
a <strong>grapheme cluster</strong>, a set of individual elements that
form a single symbol. For example, one way of representing “á” is as the
letter “a” plus an accent: <code>.</code> will match the component “a”,
while <code>\X</code> will match the complete symbol:</p>
<div class="sourceCode" id="cb9"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb9-1"><a href="#cb9-1" tabindex="-1"></a>x <span class="ot">&lt;-</span> <span class="st">&quot;a\u0301&quot;</span></span>
<span id="cb9-2"><a href="#cb9-2" tabindex="-1"></a><span class="fu">str_extract</span>(x, <span class="st">&quot;.&quot;</span>)</span>
<span id="cb9-3"><a href="#cb9-3" tabindex="-1"></a><span class="co">#&gt; [1] &quot;a&quot;</span></span>
<span id="cb9-4"><a href="#cb9-4" tabindex="-1"></a><span class="fu">str_extract</span>(x, <span class="st">&quot;</span><span class="sc">\\</span><span class="st">X&quot;</span>)</span>
<span id="cb9-5"><a href="#cb9-5" tabindex="-1"></a><span class="co">#&gt; [1] &quot;á&quot;</span></span></code></pre></div>
<p>There are five other escaped pairs that match narrower classes of
characters:</p>
<ul>
<li><p><code>\d</code>: matches any digit. The complement,
<code>\D</code>, matches any character that is not a decimal digit.</p>
<div class="sourceCode" id="cb10"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb10-1"><a href="#cb10-1" tabindex="-1"></a><span class="fu">str_extract_all</span>(<span class="st">&quot;1 + 2 = 3&quot;</span>, <span class="st">&quot;</span><span class="sc">\\</span><span class="st">d+&quot;</span>)[[<span class="dv">1</span>]]</span>
<span id="cb10-2"><a href="#cb10-2" tabindex="-1"></a><span class="co">#&gt; [1] &quot;1&quot; &quot;2&quot; &quot;3&quot;</span></span></code></pre></div>
<p>Technically, <code>\d</code> includes any character in the Unicode
Category of Nd (“Number, Decimal Digit”), which also includes numeric
symbols from other languages:</p>
<div class="sourceCode" id="cb11"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb11-1"><a href="#cb11-1" tabindex="-1"></a><span class="co"># Some Laotian numbers</span></span>
<span id="cb11-2"><a href="#cb11-2" tabindex="-1"></a><span class="fu">str_detect</span>(<span class="st">&quot;១២៣&quot;</span>, <span class="st">&quot;</span><span class="sc">\\</span><span class="st">d&quot;</span>)</span>
<span id="cb11-3"><a href="#cb11-3" tabindex="-1"></a><span class="co">#&gt; [1] TRUE</span></span></code></pre></div></li>
<li><p><code>\s</code>: matches any whitespace. This includes tabs,
newlines, form feeds, and any character in the Unicode Z Category (which
includes a variety of space characters and other separators.). The
complement, <code>\S</code>, matches any non-whitespace character.</p>
<div class="sourceCode" id="cb12"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb12-1"><a href="#cb12-1" tabindex="-1"></a>(text <span class="ot">&lt;-</span> <span class="st">&quot;Some  </span><span class="sc">\t</span><span class="st"> badly</span><span class="sc">\n\t\t</span><span class="st">spaced </span><span class="sc">\f</span><span class="st"> text&quot;</span>)</span>
<span id="cb12-2"><a href="#cb12-2" tabindex="-1"></a><span class="co">#&gt; [1] &quot;Some  \t badly\n\t\tspaced \f text&quot;</span></span>
<span id="cb12-3"><a href="#cb12-3" tabindex="-1"></a><span class="fu">str_replace_all</span>(text, <span class="st">&quot;</span><span class="sc">\\</span><span class="st">s+&quot;</span>, <span class="st">&quot; &quot;</span>)</span>
<span id="cb12-4"><a href="#cb12-4" tabindex="-1"></a><span class="co">#&gt; [1] &quot;Some badly spaced text&quot;</span></span></code></pre></div></li>
<li><p><code>\p{property name}</code> matches any character with
specific unicode property, like <code>\p{Uppercase}</code> or
<code>\p{Diacritic}</code>. The complement,
<code>\P{property name}</code>, matches all characters without the
property. A complete list of unicode properties can be found at <a href="http://www.unicode.org/reports/tr44/#Property_Index" class="uri">http://www.unicode.org/reports/tr44/#Property_Index</a>.</p>
<div class="sourceCode" id="cb13"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb13-1"><a href="#cb13-1" tabindex="-1"></a>(text <span class="ot">&lt;-</span> <span class="fu">c</span>(<span class="st">&#39;&quot;Double quotes&quot;&#39;</span>, <span class="st">&quot;«Guillemet»&quot;</span>, <span class="st">&quot;“Fancy quotes”&quot;</span>))</span>
<span id="cb13-2"><a href="#cb13-2" tabindex="-1"></a><span class="co">#&gt; [1] &quot;\&quot;Double quotes\&quot;&quot; &quot;«Guillemet»&quot;       &quot;“Fancy quotes”&quot;</span></span>
<span id="cb13-3"><a href="#cb13-3" tabindex="-1"></a><span class="fu">str_replace_all</span>(text, <span class="st">&quot;</span><span class="sc">\\</span><span class="st">p{quotation mark}&quot;</span>, <span class="st">&quot;&#39;&quot;</span>)</span>
<span id="cb13-4"><a href="#cb13-4" tabindex="-1"></a><span class="co">#&gt; [1] &quot;&#39;Double quotes&#39;&quot; &quot;&#39;Guillemet&#39;&quot;     &quot;&#39;Fancy quotes&#39;&quot;</span></span></code></pre></div></li>
<li><p><code>\w</code> matches any “word” character, which includes
alphabetic characters, marks and decimal numbers. The complement,
<code>\W</code>, matches any non-word character.</p>
<div class="sourceCode" id="cb14"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb14-1"><a href="#cb14-1" tabindex="-1"></a><span class="fu">str_extract_all</span>(<span class="st">&quot;Don&#39;t eat that!&quot;</span>, <span class="st">&quot;</span><span class="sc">\\</span><span class="st">w+&quot;</span>)[[<span class="dv">1</span>]]</span>
<span id="cb14-2"><a href="#cb14-2" tabindex="-1"></a><span class="co">#&gt; [1] &quot;Don&quot;  &quot;t&quot;    &quot;eat&quot;  &quot;that&quot;</span></span>
<span id="cb14-3"><a href="#cb14-3" tabindex="-1"></a><span class="fu">str_split</span>(<span class="st">&quot;Don&#39;t eat that!&quot;</span>, <span class="st">&quot;</span><span class="sc">\\</span><span class="st">W&quot;</span>)[[<span class="dv">1</span>]]</span>
<span id="cb14-4"><a href="#cb14-4" tabindex="-1"></a><span class="co">#&gt; [1] &quot;Don&quot;  &quot;t&quot;    &quot;eat&quot;  &quot;that&quot; &quot;&quot;</span></span></code></pre></div>
<p>Technically, <code>\w</code> also matches connector punctuation,
<code>\u200c</code> (zero width connector), and <code>\u200d</code>
(zero width joiner), but these are rarely seen in the wild.</p></li>
<li><p><code>\b</code> matches word boundaries, the transition between
word and non-word characters. <code>\B</code> matches the opposite:
boundaries that have either both word or non-word characters on either
side.</p>
<div class="sourceCode" id="cb15"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb15-1"><a href="#cb15-1" tabindex="-1"></a><span class="fu">str_replace_all</span>(<span class="st">&quot;The quick brown fox&quot;</span>, <span class="st">&quot;</span><span class="sc">\\</span><span class="st">b&quot;</span>, <span class="st">&quot;_&quot;</span>)</span>
<span id="cb15-2"><a href="#cb15-2" tabindex="-1"></a><span class="co">#&gt; [1] &quot;_The_ _quick_ _brown_ _fox_&quot;</span></span>
<span id="cb15-3"><a href="#cb15-3" tabindex="-1"></a><span class="fu">str_replace_all</span>(<span class="st">&quot;The quick brown fox&quot;</span>, <span class="st">&quot;</span><span class="sc">\\</span><span class="st">B&quot;</span>, <span class="st">&quot;_&quot;</span>)</span>
<span id="cb15-4"><a href="#cb15-4" tabindex="-1"></a><span class="co">#&gt; [1] &quot;T_h_e q_u_i_c_k b_r_o_w_n f_o_x&quot;</span></span></code></pre></div></li>
</ul>
<p>You can also create your own <strong>character classes</strong> using
<code>[]</code>:</p>
<ul>
<li><code>[abc]</code>: matches a, b, or c.</li>
<li><code>[a-z]</code>: matches every character between a and z (in
Unicode code point order).</li>
<li><code>[^abc]</code>: matches anything except a, b, or c.</li>
<li><code>[\^\-]</code>: matches <code>^</code> or <code>-</code>.</li>
</ul>
<p>There are a number of pre-built classes that you can use inside
<code>[]</code>:</p>
<ul>
<li><code>[:punct:]</code>: punctuation.</li>
<li><code>[:alpha:]</code>: letters.</li>
<li><code>[:lower:]</code>: lowercase letters.</li>
<li><code>[:upper:]</code>: upperclass letters.</li>
<li><code>[:digit:]</code>: digits.</li>
<li><code>[:xdigit:]</code>: hex digits.</li>
<li><code>[:alnum:]</code>: letters and numbers.</li>
<li><code>[:cntrl:]</code>: control characters.</li>
<li><code>[:graph:]</code>: letters, numbers, and punctuation.</li>
<li><code>[:print:]</code>: letters, numbers, punctuation, and
whitespace.</li>
<li><code>[:space:]</code>: space characters (basically equivalent to
<code>\s</code>).</li>
<li><code>[:blank:]</code>: space and tab.</li>
</ul>
<p>These all go inside the <code>[]</code> for character classes,
i.e. <code>[[:digit:]AX]</code> matches all digits, A, and X.</p>
<p>You can also using Unicode properties, like
<code>[\p{Letter}]</code>, and various set operations, like
<code>[\p{Letter}--\p{script=latin}]</code>. See
<code>?&quot;stringi-search-charclass&quot;</code> for details.</p>
</div>
<div id="alternation" class="section level2">
<h2>Alternation</h2>
<p><code>|</code> is the <strong>alternation</strong> operator, which
will pick between one or more possible matches. For example,
<code>abc|def</code> will match <code>abc</code> or
<code>def</code>:</p>
<div class="sourceCode" id="cb16"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb16-1"><a href="#cb16-1" tabindex="-1"></a><span class="fu">str_detect</span>(<span class="fu">c</span>(<span class="st">&quot;abc&quot;</span>, <span class="st">&quot;def&quot;</span>, <span class="st">&quot;ghi&quot;</span>), <span class="st">&quot;abc|def&quot;</span>)</span>
<span id="cb16-2"><a href="#cb16-2" tabindex="-1"></a><span class="co">#&gt; [1]  TRUE  TRUE FALSE</span></span></code></pre></div>
<p>Note that the precedence for <code>|</code> is low:
<code>abc|def</code> is equivalent to <code>(abc)|(def)</code> not
<code>ab(c|d)ef</code>.</p>
</div>
<div id="grouping" class="section level2">
<h2>Grouping</h2>
<p>You can use parentheses to override the default precedence rules:</p>
<div class="sourceCode" id="cb17"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb17-1"><a href="#cb17-1" tabindex="-1"></a><span class="fu">str_extract</span>(<span class="fu">c</span>(<span class="st">&quot;grey&quot;</span>, <span class="st">&quot;gray&quot;</span>), <span class="st">&quot;gre|ay&quot;</span>)</span>
<span id="cb17-2"><a href="#cb17-2" tabindex="-1"></a><span class="co">#&gt; [1] &quot;gre&quot; &quot;ay&quot;</span></span>
<span id="cb17-3"><a href="#cb17-3" tabindex="-1"></a><span class="fu">str_extract</span>(<span class="fu">c</span>(<span class="st">&quot;grey&quot;</span>, <span class="st">&quot;gray&quot;</span>), <span class="st">&quot;gr(e|a)y&quot;</span>)</span>
<span id="cb17-4"><a href="#cb17-4" tabindex="-1"></a><span class="co">#&gt; [1] &quot;grey&quot; &quot;gray&quot;</span></span></code></pre></div>
<p>Parenthesis also define “groups” that you can refer to with
<strong>backreferences</strong>, like <code>\1</code>, <code>\2</code>
etc, and can be extracted with <code>str_match()</code>. For example,
the following regular expression finds all fruits that have a repeated
pair of letters:</p>
<div class="sourceCode" id="cb18"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb18-1"><a href="#cb18-1" tabindex="-1"></a>pattern <span class="ot">&lt;-</span> <span class="st">&quot;(..)</span><span class="sc">\\</span><span class="st">1&quot;</span></span>
<span id="cb18-2"><a href="#cb18-2" tabindex="-1"></a>fruit <span class="sc">%&gt;%</span> </span>
<span id="cb18-3"><a href="#cb18-3" tabindex="-1"></a>  <span class="fu">str_subset</span>(pattern)</span>
<span id="cb18-4"><a href="#cb18-4" tabindex="-1"></a><span class="co">#&gt; [1] &quot;banana&quot;</span></span>
<span id="cb18-5"><a href="#cb18-5" tabindex="-1"></a></span>
<span id="cb18-6"><a href="#cb18-6" tabindex="-1"></a>fruit <span class="sc">%&gt;%</span> </span>
<span id="cb18-7"><a href="#cb18-7" tabindex="-1"></a>  <span class="fu">str_subset</span>(pattern) <span class="sc">%&gt;%</span> </span>
<span id="cb18-8"><a href="#cb18-8" tabindex="-1"></a>  <span class="fu">str_match</span>(pattern)</span>
<span id="cb18-9"><a href="#cb18-9" tabindex="-1"></a><span class="co">#&gt;      [,1]   [,2]</span></span>
<span id="cb18-10"><a href="#cb18-10" tabindex="-1"></a><span class="co">#&gt; [1,] &quot;anan&quot; &quot;an&quot;</span></span></code></pre></div>
<p>You can use <code>(?:...)</code>, the non-grouping parentheses, to
control precedence but not capture the match in a group. This is
slightly more efficient than capturing parentheses.</p>
<div class="sourceCode" id="cb19"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb19-1"><a href="#cb19-1" tabindex="-1"></a><span class="fu">str_match</span>(<span class="fu">c</span>(<span class="st">&quot;grey&quot;</span>, <span class="st">&quot;gray&quot;</span>), <span class="st">&quot;gr(e|a)y&quot;</span>)</span>
<span id="cb19-2"><a href="#cb19-2" tabindex="-1"></a><span class="co">#&gt;      [,1]   [,2]</span></span>
<span id="cb19-3"><a href="#cb19-3" tabindex="-1"></a><span class="co">#&gt; [1,] &quot;grey&quot; &quot;e&quot; </span></span>
<span id="cb19-4"><a href="#cb19-4" tabindex="-1"></a><span class="co">#&gt; [2,] &quot;gray&quot; &quot;a&quot;</span></span>
<span id="cb19-5"><a href="#cb19-5" tabindex="-1"></a><span class="fu">str_match</span>(<span class="fu">c</span>(<span class="st">&quot;grey&quot;</span>, <span class="st">&quot;gray&quot;</span>), <span class="st">&quot;gr(?:e|a)y&quot;</span>)</span>
<span id="cb19-6"><a href="#cb19-6" tabindex="-1"></a><span class="co">#&gt;      [,1]  </span></span>
<span id="cb19-7"><a href="#cb19-7" tabindex="-1"></a><span class="co">#&gt; [1,] &quot;grey&quot;</span></span>
<span id="cb19-8"><a href="#cb19-8" tabindex="-1"></a><span class="co">#&gt; [2,] &quot;gray&quot;</span></span></code></pre></div>
<p>This is most useful for more complex cases where you need to capture
matches and control precedence independently.</p>
</div>
<div id="anchors" class="section level2">
<h2>Anchors</h2>
<p>By default, regular expressions will match any part of a string. It’s
often useful to <strong>anchor</strong> the regular expression so that
it matches from the start or end of the string:</p>
<ul>
<li><code>^</code> matches the start of string.</li>
<li><code>$</code> matches the end of the string.</li>
</ul>
<div class="sourceCode" id="cb20"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb20-1"><a href="#cb20-1" tabindex="-1"></a>x <span class="ot">&lt;-</span> <span class="fu">c</span>(<span class="st">&quot;apple&quot;</span>, <span class="st">&quot;banana&quot;</span>, <span class="st">&quot;pear&quot;</span>)</span>
<span id="cb20-2"><a href="#cb20-2" tabindex="-1"></a><span class="fu">str_extract</span>(x, <span class="st">&quot;^a&quot;</span>)</span>
<span id="cb20-3"><a href="#cb20-3" tabindex="-1"></a><span class="co">#&gt; [1] &quot;a&quot; NA  NA</span></span>
<span id="cb20-4"><a href="#cb20-4" tabindex="-1"></a><span class="fu">str_extract</span>(x, <span class="st">&quot;a$&quot;</span>)</span>
<span id="cb20-5"><a href="#cb20-5" tabindex="-1"></a><span class="co">#&gt; [1] NA  &quot;a&quot; NA</span></span></code></pre></div>
<p>To match a literal “$” or “^”, you need to escape them,
<code>\$</code>, and <code>\^</code>.</p>
<p>For multiline strings, you can use
<code>regex(multiline = TRUE)</code>. This changes the behaviour of
<code>^</code> and <code>$</code>, and introduces three new
operators:</p>
<ul>
<li><p><code>^</code> now matches the start of each line.</p></li>
<li><p><code>$</code> now matches the end of each line.</p></li>
<li><p><code>\A</code> matches the start of the input.</p></li>
<li><p><code>\z</code> matches the end of the input.</p></li>
<li><p><code>\Z</code> matches the end of the input, but before the
final line terminator, if it exists.</p></li>
</ul>
<div class="sourceCode" id="cb21"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb21-1"><a href="#cb21-1" tabindex="-1"></a>x <span class="ot">&lt;-</span> <span class="st">&quot;Line 1</span><span class="sc">\n</span><span class="st">Line 2</span><span class="sc">\n</span><span class="st">Line 3</span><span class="sc">\n</span><span class="st">&quot;</span></span>
<span id="cb21-2"><a href="#cb21-2" tabindex="-1"></a><span class="fu">str_extract_all</span>(x, <span class="st">&quot;^Line..&quot;</span>)[[<span class="dv">1</span>]]</span>
<span id="cb21-3"><a href="#cb21-3" tabindex="-1"></a><span class="co">#&gt; [1] &quot;Line 1&quot;</span></span>
<span id="cb21-4"><a href="#cb21-4" tabindex="-1"></a><span class="fu">str_extract_all</span>(x, <span class="fu">regex</span>(<span class="st">&quot;^Line..&quot;</span>, <span class="at">multiline =</span> <span class="cn">TRUE</span>))[[<span class="dv">1</span>]]</span>
<span id="cb21-5"><a href="#cb21-5" tabindex="-1"></a><span class="co">#&gt; [1] &quot;Line 1&quot; &quot;Line 2&quot; &quot;Line 3&quot;</span></span>
<span id="cb21-6"><a href="#cb21-6" tabindex="-1"></a><span class="fu">str_extract_all</span>(x, <span class="fu">regex</span>(<span class="st">&quot;</span><span class="sc">\\</span><span class="st">ALine..&quot;</span>, <span class="at">multiline =</span> <span class="cn">TRUE</span>))[[<span class="dv">1</span>]]</span>
<span id="cb21-7"><a href="#cb21-7" tabindex="-1"></a><span class="co">#&gt; [1] &quot;Line 1&quot;</span></span></code></pre></div>
</div>
<div id="repetition" class="section level2">
<h2>Repetition</h2>
<p>You can control how many times a pattern matches with the repetition
operators:</p>
<ul>
<li><code>?</code>: 0 or 1.</li>
<li><code>+</code>: 1 or more.</li>
<li><code>*</code>: 0 or more.</li>
</ul>
<div class="sourceCode" id="cb22"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb22-1"><a href="#cb22-1" tabindex="-1"></a>x <span class="ot">&lt;-</span> <span class="st">&quot;1888 is the longest year in Roman numerals: MDCCCLXXXVIII&quot;</span></span>
<span id="cb22-2"><a href="#cb22-2" tabindex="-1"></a><span class="fu">str_extract</span>(x, <span class="st">&quot;CC?&quot;</span>)</span>
<span id="cb22-3"><a href="#cb22-3" tabindex="-1"></a><span class="co">#&gt; [1] &quot;CC&quot;</span></span>
<span id="cb22-4"><a href="#cb22-4" tabindex="-1"></a><span class="fu">str_extract</span>(x, <span class="st">&quot;CC+&quot;</span>)</span>
<span id="cb22-5"><a href="#cb22-5" tabindex="-1"></a><span class="co">#&gt; [1] &quot;CCC&quot;</span></span>
<span id="cb22-6"><a href="#cb22-6" tabindex="-1"></a><span class="fu">str_extract</span>(x, <span class="st">&#39;C[LX]+&#39;</span>)</span>
<span id="cb22-7"><a href="#cb22-7" tabindex="-1"></a><span class="co">#&gt; [1] &quot;CLXXX&quot;</span></span></code></pre></div>
<p>Note that the precedence of these operators is high, so you can
write: <code>colou?r</code> to match either American or British
spellings. That means most uses will need parentheses, like
<code>bana(na)+</code>.</p>
<p>You can also specify the number of matches precisely:</p>
<ul>
<li><code>{n}</code>: exactly n</li>
<li><code>{n,}</code>: n or more</li>
<li><code>{n,m}</code>: between n and m</li>
</ul>
<div class="sourceCode" id="cb23"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb23-1"><a href="#cb23-1" tabindex="-1"></a><span class="fu">str_extract</span>(x, <span class="st">&quot;C{2}&quot;</span>)</span>
<span id="cb23-2"><a href="#cb23-2" tabindex="-1"></a><span class="co">#&gt; [1] &quot;CC&quot;</span></span>
<span id="cb23-3"><a href="#cb23-3" tabindex="-1"></a><span class="fu">str_extract</span>(x, <span class="st">&quot;C{2,}&quot;</span>)</span>
<span id="cb23-4"><a href="#cb23-4" tabindex="-1"></a><span class="co">#&gt; [1] &quot;CCC&quot;</span></span>
<span id="cb23-5"><a href="#cb23-5" tabindex="-1"></a><span class="fu">str_extract</span>(x, <span class="st">&quot;C{2,3}&quot;</span>)</span>
<span id="cb23-6"><a href="#cb23-6" tabindex="-1"></a><span class="co">#&gt; [1] &quot;CCC&quot;</span></span></code></pre></div>
<p>By default these matches are “greedy”: they will match the longest
string possible. You can make them “lazy”, matching the shortest string
possible by putting a <code>?</code> after them:</p>
<ul>
<li><code>??</code>: 0 or 1, prefer 0.</li>
<li><code>+?</code>: 1 or more, match as few times as possible.</li>
<li><code>*?</code>: 0 or more, match as few times as possible.</li>
<li><code>{n,}?</code>: n or more, match as few times as possible.</li>
<li><code>{n,m}?</code>: between n and m, , match as few times as
possible, but at least n.</li>
</ul>
<div class="sourceCode" id="cb24"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb24-1"><a href="#cb24-1" tabindex="-1"></a><span class="fu">str_extract</span>(x, <span class="fu">c</span>(<span class="st">&quot;C{2,3}&quot;</span>, <span class="st">&quot;C{2,3}?&quot;</span>))</span>
<span id="cb24-2"><a href="#cb24-2" tabindex="-1"></a><span class="co">#&gt; [1] &quot;CCC&quot; &quot;CC&quot;</span></span>
<span id="cb24-3"><a href="#cb24-3" tabindex="-1"></a><span class="fu">str_extract</span>(x, <span class="fu">c</span>(<span class="st">&quot;C[LX]+&quot;</span>, <span class="st">&quot;C[LX]+?&quot;</span>))</span>
<span id="cb24-4"><a href="#cb24-4" tabindex="-1"></a><span class="co">#&gt; [1] &quot;CLXXX&quot; &quot;CL&quot;</span></span></code></pre></div>
<p>You can also make the matches possessive by putting a <code>+</code>
after them, which means that if later parts of the match fail, the
repetition will not be re-tried with a smaller number of characters.
This is an advanced feature used to improve performance in worst-case
scenarios (called “catastrophic backtracking”).</p>
<ul>
<li><code>?+</code>: 0 or 1, possessive.</li>
<li><code>++</code>: 1 or more, possessive.</li>
<li><code>*+</code>: 0 or more, possessive.</li>
<li><code>{n}+</code>: exactly n, possessive.</li>
<li><code>{n,}+</code>: n or more, possessive.</li>
<li><code>{n,m}+</code>: between n and m, possessive.</li>
</ul>
<p>A related concept is the <strong>atomic-match</strong> parenthesis,
<code>(?&gt;...)</code>. If a later match fails and the engine needs to
back-track, an atomic match is kept as is: it succeeds or fails as a
whole. Compare the following two regular expressions:</p>
<div class="sourceCode" id="cb25"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb25-1"><a href="#cb25-1" tabindex="-1"></a><span class="fu">str_detect</span>(<span class="st">&quot;ABC&quot;</span>, <span class="st">&quot;(?&gt;A|.B)C&quot;</span>)</span>
<span id="cb25-2"><a href="#cb25-2" tabindex="-1"></a><span class="co">#&gt; [1] FALSE</span></span>
<span id="cb25-3"><a href="#cb25-3" tabindex="-1"></a><span class="fu">str_detect</span>(<span class="st">&quot;ABC&quot;</span>, <span class="st">&quot;(?:A|.B)C&quot;</span>)</span>
<span id="cb25-4"><a href="#cb25-4" tabindex="-1"></a><span class="co">#&gt; [1] TRUE</span></span></code></pre></div>
<p>The atomic match fails because it matches A, and then the next
character is a C so it fails. The regular match succeeds because it
matches A, but then C doesn’t match, so it back-tracks and tries B
instead.</p>
</div>
<div id="look-arounds" class="section level2">
<h2>Look arounds</h2>
<p>These assertions look ahead or behind the current match without
“consuming” any characters (i.e. changing the input position).</p>
<ul>
<li><p><code>(?=...)</code>: positive look-ahead assertion. Matches if
<code>...</code> matches at the current input.</p></li>
<li><p><code>(?!...)</code>: negative look-ahead assertion. Matches if
<code>...</code> <strong>does not</strong> match at the current
input.</p></li>
<li><p><code>(?&lt;=...)</code>: positive look-behind assertion. Matches
if <code>...</code> matches text preceding the current position, with
the last character of the match being the character just before the
current position. Length must be bounded<br />
(i.e. no <code>*</code> or <code>+</code>).</p></li>
<li><p><code>(?&lt;!...)</code>: negative look-behind assertion. Matches
if <code>...</code> <strong>does not</strong> match text preceding the
current position. Length must be bounded<br />
(i.e. no <code>*</code> or <code>+</code>).</p></li>
</ul>
<p>These are useful when you want to check that a pattern exists, but
you don’t want to include it in the result:</p>
<div class="sourceCode" id="cb26"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb26-1"><a href="#cb26-1" tabindex="-1"></a>x <span class="ot">&lt;-</span> <span class="fu">c</span>(<span class="st">&quot;1 piece&quot;</span>, <span class="st">&quot;2 pieces&quot;</span>, <span class="st">&quot;3&quot;</span>)</span>
<span id="cb26-2"><a href="#cb26-2" tabindex="-1"></a><span class="fu">str_extract</span>(x, <span class="st">&quot;</span><span class="sc">\\</span><span class="st">d+(?= pieces?)&quot;</span>)</span>
<span id="cb26-3"><a href="#cb26-3" tabindex="-1"></a><span class="co">#&gt; [1] &quot;1&quot; &quot;2&quot; NA</span></span>
<span id="cb26-4"><a href="#cb26-4" tabindex="-1"></a></span>
<span id="cb26-5"><a href="#cb26-5" tabindex="-1"></a>y <span class="ot">&lt;-</span> <span class="fu">c</span>(<span class="st">&quot;100&quot;</span>, <span class="st">&quot;$400&quot;</span>)</span>
<span id="cb26-6"><a href="#cb26-6" tabindex="-1"></a><span class="fu">str_extract</span>(y, <span class="st">&quot;(?&lt;=</span><span class="sc">\\</span><span class="st">$)</span><span class="sc">\\</span><span class="st">d+&quot;</span>)</span>
<span id="cb26-7"><a href="#cb26-7" tabindex="-1"></a><span class="co">#&gt; [1] NA    &quot;400&quot;</span></span></code></pre></div>
</div>
<div id="comments" class="section level2">
<h2>Comments</h2>
<p>There are two ways to include comments in a regular expression. The
first is with <code>(?#...)</code>:</p>
<div class="sourceCode" id="cb27"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb27-1"><a href="#cb27-1" tabindex="-1"></a><span class="fu">str_detect</span>(<span class="st">&quot;xyz&quot;</span>, <span class="st">&quot;x(?#this is a comment)&quot;</span>)</span>
<span id="cb27-2"><a href="#cb27-2" tabindex="-1"></a><span class="co">#&gt; [1] TRUE</span></span></code></pre></div>
<p>The second is to use <code>regex(comments = TRUE)</code>. This form
ignores spaces and newlines, and anything everything after
<code>#</code>. To match a literal space, you’ll need to escape it:
<code>&quot;\\ &quot;</code>. This is a useful way of describing complex regular
expressions:</p>
<div class="sourceCode" id="cb28"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb28-1"><a href="#cb28-1" tabindex="-1"></a>phone <span class="ot">&lt;-</span> <span class="fu">regex</span>(<span class="st">&quot;</span></span>
<span id="cb28-2"><a href="#cb28-2" tabindex="-1"></a><span class="st">  </span><span class="sc">\\</span><span class="st">(?       # optional opening parens</span></span>
<span id="cb28-3"><a href="#cb28-3" tabindex="-1"></a><span class="st">  (</span><span class="sc">\\</span><span class="st">d{3})   # area code</span></span>
<span id="cb28-4"><a href="#cb28-4" tabindex="-1"></a><span class="st">  </span><span class="sc">\\</span><span class="st">)?       # optional closing parens</span></span>
<span id="cb28-5"><a href="#cb28-5" tabindex="-1"></a><span class="st">  (?:-|</span><span class="sc">\\</span><span class="st"> )? # optional dash or space</span></span>
<span id="cb28-6"><a href="#cb28-6" tabindex="-1"></a><span class="st">  (</span><span class="sc">\\</span><span class="st">d{3})   # another three numbers</span></span>
<span id="cb28-7"><a href="#cb28-7" tabindex="-1"></a><span class="st">  (?:-|</span><span class="sc">\\</span><span class="st"> )? # optional dash or space</span></span>
<span id="cb28-8"><a href="#cb28-8" tabindex="-1"></a><span class="st">  (</span><span class="sc">\\</span><span class="st">d{3})   # three more numbers</span></span>
<span id="cb28-9"><a href="#cb28-9" tabindex="-1"></a><span class="st">  &quot;</span>, <span class="at">comments =</span> <span class="cn">TRUE</span>)</span>
<span id="cb28-10"><a href="#cb28-10" tabindex="-1"></a></span>
<span id="cb28-11"><a href="#cb28-11" tabindex="-1"></a><span class="fu">str_match</span>(<span class="fu">c</span>(<span class="st">&quot;514-791-8141&quot;</span>, <span class="st">&quot;(514) 791 8141&quot;</span>), phone)</span>
<span id="cb28-12"><a href="#cb28-12" tabindex="-1"></a><span class="co">#&gt;      [,1]            [,2]  [,3]  [,4] </span></span>
<span id="cb28-13"><a href="#cb28-13" tabindex="-1"></a><span class="co">#&gt; [1,] &quot;514-791-814&quot;   &quot;514&quot; &quot;791&quot; &quot;814&quot;</span></span>
<span id="cb28-14"><a href="#cb28-14" tabindex="-1"></a><span class="co">#&gt; [2,] &quot;(514) 791 814&quot; &quot;514&quot; &quot;791&quot; &quot;814&quot;</span></span></code></pre></div>
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