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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>9.1 Writing Regexp Patterns</title><link rel="stylesheet" type="text/css" href="../scribble.css" title="default"/><link rel="stylesheet" type="text/css" href="../racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-style.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../doc-site.css" title="default"/><script type="text/javascript" src="../scribble-common.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../doc-site.js"></script><script type="text/javascript" src="../local-redirect/local-redirect.js"></script><script type="text/javascript" src="../local-redirect/local-user-redirect.js"></script><!--[if IE 6]><style type="text/css">.SIEHidden { overflow: hidden; }</style><![endif]--></head><body id="doc-racket-lang-org"><div class="tocset"><div class="tocview"><div class="tocviewlist tocviewlisttopspace"><div class="tocviewtitle"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_0");">►</a></td><td></td><td><a href="index.html" class="tocviewlink" data-pltdoc="x"><span style="font-weight: bold">The Racket Guide</span></a></td></tr></table></div><div class="tocviewsublisttop" style="display: none;" id="tocview_0"><table cellspacing="0" cellpadding="0"><tr><td align="right">1 </td><td><a href="intro.html" class="tocviewlink" data-pltdoc="x">Welcome to Racket</a></td></tr><tr><td align="right">2 </td><td><a href="to-scheme.html" class="tocviewlink" data-pltdoc="x">Racket Essentials</a></td></tr><tr><td align="right">3 </td><td><a href="datatypes.html" class="tocviewlink" data-pltdoc="x">Built-<wbr></wbr>In Datatypes</a></td></tr><tr><td align="right">4 </td><td><a href="scheme-forms.html" class="tocviewlink" data-pltdoc="x">Expressions and Definitions</a></td></tr><tr><td align="right">5 </td><td><a href="define-struct.html" class="tocviewlink" data-pltdoc="x">Programmer-<wbr></wbr>Defined Datatypes</a></td></tr><tr><td align="right">6 </td><td><a href="modules.html" class="tocviewlink" data-pltdoc="x">Modules</a></td></tr><tr><td align="right">7 </td><td><a href="contracts.html" class="tocviewlink" data-pltdoc="x">Contracts</a></td></tr><tr><td align="right">8 </td><td><a href="i_o.html" class="tocviewlink" data-pltdoc="x">Input and Output</a></td></tr><tr><td align="right">9 </td><td><a href="regexp.html" class="tocviewselflink" data-pltdoc="x">Regular Expressions</a></td></tr><tr><td align="right">10 </td><td><a href="control.html" class="tocviewlink" data-pltdoc="x">Exceptions and Control</a></td></tr><tr><td align="right">11 </td><td><a href="for.html" class="tocviewlink" data-pltdoc="x">Iterations and Comprehensions</a></td></tr><tr><td align="right">12 </td><td><a href="match.html" class="tocviewlink" data-pltdoc="x">Pattern Matching</a></td></tr><tr><td align="right">13 </td><td><a href="classes.html" class="tocviewlink" data-pltdoc="x">Classes and Objects</a></td></tr><tr><td align="right">14 </td><td><a href="units.html" class="tocviewlink" data-pltdoc="x">Units</a></td></tr><tr><td align="right">15 </td><td><a href="reflection.html" class="tocviewlink" data-pltdoc="x">Reflection and Dynamic Evaluation</a></td></tr><tr><td align="right">16 </td><td><a href="macros.html" class="tocviewlink" data-pltdoc="x">Macros</a></td></tr><tr><td align="right">17 </td><td><a href="languages.html" class="tocviewlink" data-pltdoc="x">Creating Languages</a></td></tr><tr><td align="right">18 </td><td><a href="concurrency.html" class="tocviewlink" data-pltdoc="x">Concurrency and Synchronization</a></td></tr><tr><td align="right">19 </td><td><a href="performance.html" class="tocviewlink" data-pltdoc="x">Performance</a></td></tr><tr><td align="right">20 </td><td><a href="parallelism.html" class="tocviewlink" data-pltdoc="x">Parallelism</a></td></tr><tr><td align="right">21 </td><td><a href="running.html" class="tocviewlink" data-pltdoc="x">Running and Creating Executables</a></td></tr><tr><td align="right">22 </td><td><a href="More_Libraries.html" class="tocviewlink" data-pltdoc="x">More Libraries</a></td></tr><tr><td align="right">23 </td><td><a href="dialects.html" class="tocviewlink" data-pltdoc="x">Dialects of Racket and Scheme</a></td></tr><tr><td align="right">24 </td><td><a href="other-editors.html" class="tocviewlink" data-pltdoc="x">Command-<wbr></wbr>Line Tools and Your Editor of Choice</a></td></tr><tr><td align="right"></td><td><a href="doc-bibliography.html" class="tocviewlink" data-pltdoc="x">Bibliography</a></td></tr><tr><td align="right"></td><td><a href="doc-index.html" class="tocviewlink" data-pltdoc="x">Index</a></td></tr></table></div></div><div class="tocviewlist"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_1");">▼</a></td><td>9 </td><td><a href="regexp.html" class="tocviewlink" data-pltdoc="x">Regular Expressions</a></td></tr></table><div class="tocviewsublistbottom" style="display: block;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">9.1 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Writing Regexp Patterns</a></td></tr><tr><td align="right">9.2 </td><td><a href="regexp-match.html" class="tocviewlink" data-pltdoc="x">Matching Regexp Patterns</a></td></tr><tr><td align="right">9.3 </td><td><a href="regexp-assert.html" class="tocviewlink" data-pltdoc="x">Basic Assertions</a></td></tr><tr><td align="right">9.4 </td><td><a href="regexp-chars.html" class="tocviewlink" data-pltdoc="x">Characters and Character Classes</a></td></tr><tr><td align="right">9.5 </td><td><a href="regexp-quant.html" class="tocviewlink" data-pltdoc="x">Quantifiers</a></td></tr><tr><td align="right">9.6 </td><td><a href="regexp-clusters.html" class="tocviewlink" data-pltdoc="x">Clusters</a></td></tr><tr><td align="right">9.7 </td><td><a href="regexp-alternation.html" class="tocviewlink" data-pltdoc="x">Alternation</a></td></tr><tr><td align="right">9.8 </td><td><a href="Backtracking.html" class="tocviewlink" data-pltdoc="x">Backtracking</a></td></tr><tr><td align="right">9.9 </td><td><a href="Looking_Ahead_and_Behind.html" class="tocviewlink" data-pltdoc="x">Looking Ahead and Behind</a></td></tr><tr><td align="right">9.10 </td><td><a href="An_Extended_Example.html" class="tocviewlink" data-pltdoc="x">An Extended Example</a></td></tr></table></div></div></div></div><div class="maincolumn"><div class="main"><div class="navsettop"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.7", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.7");">top</a></span><span class="navright"> <a href="regexp.html" title="backward to "9 Regular Expressions"" data-pltdoc="x">← prev</a> <a href="regexp.html" title="up to "9 Regular Expressions"" data-pltdoc="x">up</a> <a href="regexp-match.html" title="forward to "9.2 Matching Regexp Patterns"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-source-pkg="racket-doc" x-part-tag=""regexp-intro"">9.1<tt> </tt><a name="(part._regexp-intro)"></a>Writing Regexp Patterns</h4><p>A string or <a href="bytestrings.html#%28tech._byte._string%29" class="techoutside" data-pltdoc="x"><span class="techinside">byte string</span></a> can be used directly as a <a href="regexp.html#%28tech._regexp%29" class="techoutside" data-pltdoc="x"><span class="techinside">regexp</span></a>
pattern, or it can be prefixed with <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#rx</span><span class="hspace"></span></span> to form a literal
<a href="regexp.html#%28tech._regexp%29" class="techoutside" data-pltdoc="x"><span class="techinside">regexp</span></a> value. For example, <span class="RktVal">#rx"abc"</span> is a string-based
<a href="regexp.html#%28tech._regexp%29" class="techoutside" data-pltdoc="x"><span class="techinside">regexp</span></a> value, and <span class="RktVal">#rx#"abc"</span> is a <a href="bytestrings.html#%28tech._byte._string%29" class="techoutside" data-pltdoc="x"><span class="techinside">byte
string</span></a>-based <a href="regexp.html#%28tech._regexp%29" class="techoutside" data-pltdoc="x"><span class="techinside">regexp</span></a> value. Alternately, a string or byte
string can be prefixed with <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#px</span><span class="hspace"></span></span>, as in <span class="RktVal">#px"abc"</span>,
for a slightly extended syntax of patterns within the string.</p><p>Most of the characters in a <a href="regexp.html#%28tech._regexp%29" class="techoutside" data-pltdoc="x"><span class="techinside">regexp</span></a> pattern are meant to match
occurrences of themselves in the <a href="regexp.html#%28tech._text._string%29" class="techoutside" data-pltdoc="x"><span class="techinside">text string</span></a>. Thus, the pattern
<span class="RktVal">#rx"abc"</span> matches a string that contains the characters
<span class="RktInBG"><span class="hspace"></span><span class="RktIn">a</span><span class="hspace"></span></span>, <span class="RktInBG"><span class="hspace"></span><span class="RktIn">b</span><span class="hspace"></span></span>, and <span class="RktInBG"><span class="hspace"></span><span class="RktIn">c</span><span class="hspace"></span></span> in succession. Other
characters act as <a name="(tech._metacharacter)"></a><span style="font-style: italic">metacharacters</span>, and some character
sequences act as <a name="(tech._metasequence)"></a><span style="font-style: italic">metasequences</span>. That is, they specify
something other than their literal selves. For example, in the
pattern <span class="RktVal">#rx"a.c"</span>, the characters <span class="RktInBG"><span class="hspace"></span><span class="RktIn">a</span><span class="hspace"></span></span> and <span class="RktInBG"><span class="hspace"></span><span class="RktIn">c</span><span class="hspace"></span></span>
stand for themselves, but the <a href="#%28tech._metacharacter%29" class="techoutside" data-pltdoc="x"><span class="techinside">metacharacter</span></a> <span class="RktInBG"><span class="hspace"></span><span class="RktIn">.</span><span class="hspace"></span></span> can
match <span style="font-style: italic">any</span> character. Therefore, the pattern <span class="RktVal">#rx"a.c"</span>
matches an <span class="RktInBG"><span class="hspace"></span><span class="RktIn">a</span><span class="hspace"></span></span>, any character, and <span class="RktInBG"><span class="hspace"></span><span class="RktIn">c</span><span class="hspace"></span></span> in succession.</p><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p>When we want a literal <span class="RktInBG"><span class="hspace"></span><span class="RktIn">\</span><span class="hspace"></span></span> inside a Racket string
or regexp literal, we must escape it so that it shows up in the string
at all. Racket strings use <span class="RktInBG"><span class="hspace"></span><span class="RktIn">\</span><span class="hspace"></span></span> as the escape character, so we
end up with two <span class="RktInBG"><span class="hspace"></span><span class="RktIn">\</span><span class="hspace"></span></span>s: one Racket-string <span class="RktInBG"><span class="hspace"></span><span class="RktIn">\</span><span class="hspace"></span></span> to escape
the regexp <span class="RktInBG"><span class="hspace"></span><span class="RktIn">\</span><span class="hspace"></span></span>, which then escapes the <span class="RktInBG"><span class="hspace"></span><span class="RktIn">.</span><span class="hspace"></span></span>. Another
character that would need escaping inside a Racket string is
<span class="RktInBG"><span class="hspace"></span><span class="RktIn">"</span><span class="hspace"></span></span>.</p></blockquote></blockquote></blockquote><p>If we needed to match the character <span class="RktInBG"><span class="hspace"></span><span class="RktIn">.</span><span class="hspace"></span></span> itself, we can escape
it by precede it with a <span class="RktInBG"><span class="hspace"></span><span class="RktIn">\</span><span class="hspace"></span></span>. The character sequence
<span class="RktInBG"><span class="hspace"></span><span class="RktIn">\.</span><span class="hspace"></span></span> is thus a <a href="#%28tech._metasequence%29" class="techoutside" data-pltdoc="x"><span class="techinside">metasequence</span></a>, since it doesn’t match
itself but rather just <span class="RktInBG"><span class="hspace"></span><span class="RktIn">.</span><span class="hspace"></span></span>. So, to match <span class="RktInBG"><span class="hspace"></span><span class="RktIn">a</span><span class="hspace"></span></span>,
<span class="RktInBG"><span class="hspace"></span><span class="RktIn">.</span><span class="hspace"></span></span>, and <span class="RktInBG"><span class="hspace"></span><span class="RktIn">c</span><span class="hspace"></span></span> in succession, we use the regexp pattern
<span class="RktVal">#rx"a\\.c"</span>; the double <span class="RktInBG"><span class="hspace"></span><span class="RktIn">\</span><span class="hspace"></span></span> is an artifact of Racket
strings, not the <a href="regexp.html#%28tech._regexp%29" class="techoutside" data-pltdoc="x"><span class="techinside">regexp</span></a> pattern itself.</p><p>The <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">regexp</a></span> function takes a string or byte string and
produces a <a href="regexp.html#%28tech._regexp%29" class="techoutside" data-pltdoc="x"><span class="techinside">regexp</span></a> value. Use <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">regexp</a></span> when you construct
a pattern to be matched against multiple strings, since a pattern is
compiled to a <a href="regexp.html#%28tech._regexp%29" class="techoutside" data-pltdoc="x"><span class="techinside">regexp</span></a> value before it can be used in a match.
The <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._pregexp%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">pregexp</a></span> function is like <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">regexp</a></span>, but using the
extended syntax. Regexp values as literals with <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#rx</span><span class="hspace"></span></span> or
<span class="RktInBG"><span class="hspace"></span><span class="RktIn">#px</span><span class="hspace"></span></span> are compiled once and for all when they are read.</p><p>The <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._regexp-quote%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">regexp-quote</a></span> function takes an arbitrary string and
returns a string for a pattern that matches exactly the original
string. In particular, characters in the input string that could serve
as regexp metacharacters are escaped with a backslash, so that they
safely match only themselves.</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._regexp-quote%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">regexp-quote</a></span><span class="hspace"> </span><span class="RktVal">"cons"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">"cons"</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._regexp-quote%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">regexp-quote</a></span><span class="hspace"> </span><span class="RktVal">"list?"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">"list\\?"</span></p></td></tr></table></blockquote><p>The <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._regexp-quote%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">regexp-quote</a></span> function is useful when building a composite
<a href="regexp.html#%28tech._regexp%29" class="techoutside" data-pltdoc="x"><span class="techinside">regexp</span></a> from a mix of <a href="regexp.html#%28tech._regexp%29" class="techoutside" data-pltdoc="x"><span class="techinside">regexp</span></a> strings and verbatim strings.</p><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.7", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.7");">top</a></span><span class="navright"> <a href="regexp.html" title="backward to "9 Regular Expressions"" data-pltdoc="x">← prev</a> <a href="regexp.html" title="up to "9 Regular Expressions"" data-pltdoc="x">up</a> <a href="regexp-match.html" title="forward to "9.2 Matching Regexp Patterns"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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