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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>9.6 Clusters</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 (Components)</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="tocviewsublist" style="display: block;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">9.1 </td><td><a href="regexp-intro.html" class="tocviewlink" 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="" class="tocviewselflink" 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 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_2");">►</a></td><td>9.6 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Clusters</a></td></tr></table><div class="tocviewsublistbottom" style="display: none;" id="tocview_2"><table cellspacing="0" cellpadding="0"><tr><td align="right">9.6.1 </td><td><a href="#%28part._.Backreferences%29" class="tocviewlink" data-pltdoc="x">Backreferences</a></td></tr><tr><td align="right">9.6.2 </td><td><a href="#%28part._.Non-capturing_.Clusters%29" class="tocviewlink" data-pltdoc="x">Non-<wbr></wbr>capturing Clusters</a></td></tr><tr><td align="right">9.6.3 </td><td><a href="#%28part._regexp-cloister%29" class="tocviewlink" data-pltdoc="x">Cloisters</a></td></tr></table></div></div></div><div class="tocsub"><div class="tocsubtitle">On this page:</div><table class="tocsublist" cellspacing="0"><tr><td><span class="tocsublinknumber">9.6.1<tt> </tt></span><a href="#%28part._.Backreferences%29" class="tocsubseclink" data-pltdoc="x">Backreferences</a></td></tr><tr><td><span class="tocsublinknumber">9.6.2<tt> </tt></span><a href="#%28part._.Non-capturing_.Clusters%29" class="tocsubseclink" data-pltdoc="x">Non-<wbr></wbr>capturing Clusters</a></td></tr><tr><td><span class="tocsublinknumber">9.6.3<tt> </tt></span><a href="#%28part._regexp-cloister%29" class="tocsubseclink" data-pltdoc="x">Cloisters</a></td></tr></table></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, "6.1", "../");" 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("6.1");">top</a></span><span class="navright"> <a href="regexp-quant.html" title="backward to "9.5 Quantifiers"" data-pltdoc="x">← prev</a> <a href="regexp.html" title="up to "9 Regular Expressions"" data-pltdoc="x">up</a> <a href="regexp-alternation.html" title="forward to "9.7 Alternation"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-part-tag=""regexp-clusters"">9.6<tt> </tt><a name="(part._regexp-clusters)"></a>Clusters</h4><p><a name="(tech._clustering)"></a><span style="font-style: italic">Clustering</span>—<wbr></wbr>enclosure within parens
<span class="RktInBG"><span class="hspace"></span><span class="RktIn">(</span><span class="hspace"></span></span>...<span class="RktInBG"><span class="hspace"></span><span class="RktIn">)</span><span class="hspace"></span></span>—<wbr></wbr>identifies the enclosed
<a name="(tech._subpattern)"></a><span style="font-style: italic">subpattern</span> as a single entity. It causes the matcher to
capture the <a name="(tech._submatch)"></a><span style="font-style: italic">submatch</span>, or the portion of the string matching
the subpattern, in addition to the overall match:</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktVal">#rx"([a-z]+) ([0-9]+), ([0-9]+)"</span><span class="hspace"> </span><span class="RktVal">"jan 1, 1970"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'("jan 1, 1970" "jan" "1" "1970")</span></p></td></tr></table></blockquote><p>Clustering also causes a following quantifier to treat the entire
enclosed subpattern as an entity:</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktVal">#rx"(pu )*"</span><span class="hspace"> </span><span class="RktVal">"pu pu platter"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'("pu pu " "pu ")</span></p></td></tr></table></blockquote><p>The number of submatches returned is always equal to the number of
subpatterns specified in the regexp, even if a particular subpattern
happens to match more than one substring or no substring at all.</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktVal">#rx"([a-z ]+;)*"</span><span class="hspace"> </span><span class="RktVal">"lather; rinse; repeat;"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'("lather; rinse; repeat;" " repeat;")</span></p></td></tr></table></blockquote><p>Here, the <span class="RktInBG"><span class="hspace"></span><span class="RktIn">*</span><span class="hspace"></span></span>-quantified subpattern matches three times, but
it is the last submatch that is returned.</p><p>It is also possible for a quantified subpattern to fail to match, even
if the overall pattern matches. In such cases, the failing submatch
is represented by <span class="RktVal">#f</span></p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">date-re</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">match </span><span class="RktCmt">‘</span><span class="RktCmt">month year</span><span class="RktCmt">'</span><span class="RktCmt"> or </span><span class="RktCmt">‘</span><span class="RktCmt">month day, year</span><span class="RktCmt">'</span><span class="RktCmt">;</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">subpattern matches day, if present</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">#rx"([a-z]+) +([0-9]+,)? *([0-9]+)"</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0"><tr><td></td></tr></table></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktSym">date-re</span><span class="hspace"> </span><span class="RktVal">"jan 1, 1970"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'("jan 1, 1970" "jan" "1," "1970")</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktSym">date-re</span><span class="hspace"> </span><span class="RktVal">"jan 1970"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'("jan 1970" "jan" #f "1970")</span></p></td></tr></table></blockquote><h5 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-part-tag=""Backreferences"">9.6.1<tt> </tt><a name="(part._.Backreferences)"></a>Backreferences</h5><p><a href="#%28tech._submatch%29" class="techoutside" data-pltdoc="x"><span class="techinside">Submatch</span></a>es can be used in the insert string argument of the
procedures <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-replace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-replace</a></span> and <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._regexp-replace%252A%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-replace*</a></span>. The
insert string can use <span class="RktInBG"><span class="hspace"></span><span class="RktIn">\</span><span class="hspace"></span></span><span style="font-style: italic">n</span> as a <a name="(tech._backreference)"></a><span style="font-style: italic">backreference</span>
to refer back to the <span style="font-style: italic">n</span>th submatch, which is the substring
that matched the <span style="font-style: italic">n</span>th subpattern. A <span class="RktInBG"><span class="hspace"></span><span class="RktIn">\0</span><span class="hspace"></span></span> refers to the
entire match, and it can also be specified as <span class="RktInBG"><span class="hspace"></span><span class="RktIn">\&</span><span class="hspace"></span></span>.</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-replace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-replace</a></span><span class="hspace"> </span><span class="RktVal">#rx"_(.+?)_"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"the _nina_, the _pinta_, and the _santa maria_"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"*\\1*"</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">"the *nina*, the _pinta_, and the _santa maria_"</span></p></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._regexp-replace%252A%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-replace*</a></span><span class="hspace"> </span><span class="RktVal">#rx"_(.+?)_"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"the _nina_, the _pinta_, and the _santa maria_"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"*\\1*"</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">"the *nina*, the *pinta*, and the *santa maria*"</span></p></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-replace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-replace</a></span><span class="hspace"> </span><span class="RktVal">#px"(\\S+) (\\S+) (\\S+)"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"eat to live"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"\\3 \\2 \\1"</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">"live to eat"</span></p></td></tr></table></blockquote><p>Use <span class="RktInBG"><span class="hspace"></span><span class="RktIn">\\</span><span class="hspace"></span></span> in the insert string to specify a literal backslash.
Also, <span class="RktInBG"><span class="hspace"></span><span class="RktIn">\$</span><span class="hspace"></span></span> stands for an empty string, and is useful for
separating a backreference <span class="RktInBG"><span class="hspace"></span><span class="RktIn">\</span><span class="hspace"></span></span><span style="font-style: italic">n</span> from an immediately
following number.</p><p>Backreferences can also be used within a <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#px</span><span class="hspace"></span></span> pattern to
refer back to an already matched subpattern in the pattern.
<span class="RktInBG"><span class="hspace"></span><span class="RktIn">\</span><span class="hspace"></span></span><span style="font-style: italic">n</span> stands for an exact repeat of the <span style="font-style: italic">n</span>th
submatch. Note that <span class="RktInBG"><span class="hspace"></span><span class="RktIn">\0</span><span class="hspace"></span></span>, which is useful in an insert string,
makes no sense within the regexp pattern, because the entire regexp
has not matched yet so you cannot refer back to it.}</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktVal">#px"([a-z]+) and \\1"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"billions and billions"</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'("billions and billions" "billions")</span></p></td></tr></table></blockquote><p>Note that the <a href="#%28tech._backreference%29" class="techoutside" data-pltdoc="x"><span class="techinside">backreference</span></a> is not simply a repeat of the
previous subpattern. Rather it is a repeat of the particular
substring already matched by the subpattern.</p><p>In the above example, the <a href="#%28tech._backreference%29" class="techoutside" data-pltdoc="x"><span class="techinside">backreference</span></a> can only match
<span class="RktInBG"><span class="hspace"></span><span class="RktIn">billions</span><span class="hspace"></span></span>. It will not match <span class="RktInBG"><span class="hspace"></span><span class="RktIn">millions</span><span class="hspace"></span></span>, even though
the subpattern it harks back to—<wbr></wbr><span class="RktInBG"><span class="hspace"></span><span class="RktIn">([a-z]+)</span><span class="hspace"></span></span>—<wbr></wbr>would have had
no problem doing so:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktVal">#px"([a-z]+) and \\1"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"billions and millions"</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">#f</span></p></td></tr></table></blockquote><p>The following example marks all immediately repeating patterns in a
number string:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._regexp-replace%252A%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-replace*</a></span><span class="hspace"> </span><span class="RktVal">#px"(\\d+)\\1"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"123340983242432420980980234"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"{\\1,\\1}"</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">"12{3,3}40983{24,24}3242{098,098}0234"</span></p></td></tr></table></blockquote><p>The following example corrects doubled words:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._regexp-replace%252A%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-replace*</a></span><span class="hspace"> </span><span class="RktVal">#px"\\b(\\S+) \\1\\b"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=strings.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._string-append%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">string-append</a></span><span class="hspace"> </span><span class="RktVal">"now is the the time for all good men to "</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"to come to the aid of of the party"</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"\\1"</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">"now is the time for all good men to come to the aid of the party"</span></p></td></tr></table></blockquote><h5 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-part-tag=""Non-capturing_Clusters"">9.6.2<tt> </tt><a name="(part._.Non-capturing_.Clusters)"></a>Non-capturing Clusters</h5><p>It is often required to specify a cluster (typically for
quantification) but without triggering the capture of <a href="#%28tech._submatch%29" class="techoutside" data-pltdoc="x"><span class="techinside">submatch</span></a>
information. Such clusters are called <a name="(tech._non._capturing)"></a><span style="font-style: italic">non-capturing</span>. To
create a non-capturing cluster, use <span class="RktInBG"><span class="hspace"></span><span class="RktIn">(?:</span><span class="hspace"></span></span> instead of
<span class="RktInBG"><span class="hspace"></span><span class="RktIn">(</span><span class="hspace"></span></span> as the cluster opener.</p><p>In the following example, a non-capturing cluster eliminates the
“directory” portion of a given Unix pathname, and a capturing
cluster identifies the basename.</p><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p>But don’t parse paths with regexps. Use functions like
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Manipulating_Paths.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._split-path%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">split-path</a></span>, instead.</p></blockquote></blockquote></blockquote><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktVal">#rx"^(?:[a-z]*/)*([a-z]+)$"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"/usr/local/bin/racket"</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'("/usr/local/bin/racket" "racket")</span></p></td></tr></table></blockquote><h5 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-part-tag=""regexp-cloister"">9.6.3<tt> </tt><a name="(part._regexp-cloister)"></a>Cloisters</h5><p>The location between the <span class="RktInBG"><span class="hspace"></span><span class="RktIn">?</span><span class="hspace"></span></span> and the <span class="RktInBG"><span class="hspace"></span><span class="RktIn">:</span><span class="hspace"></span></span> of a
non-capturing cluster is called a <a name="(tech._cloister)"></a><span style="font-style: italic">cloister</span>. You can put
modifiers there that will cause the enclustered <a href="#%28tech._subpattern%29" class="techoutside" data-pltdoc="x"><span class="techinside">subpattern</span></a> to
be treated specially. The modifier <span class="RktInBG"><span class="hspace"></span><span class="RktIn">i</span><span class="hspace"></span></span> causes the subpattern
to match case-insensitively:</p><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p>The term <span style="font-style: italic">cloister</span> is a useful, if terminally
cute, coinage from the abbots of Perl.</p></blockquote></blockquote></blockquote><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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktVal">#rx"(?i:hearth)"</span><span class="hspace"> </span><span class="RktVal">"HeartH"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'("HeartH")</span></p></td></tr></table></blockquote><p>The modifier <span class="RktInBG"><span class="hspace"></span><span class="RktIn">m</span><span class="hspace"></span></span> causes the <a href="#%28tech._subpattern%29" class="techoutside" data-pltdoc="x"><span class="techinside">subpattern</span></a> to match in
<a name="(tech._multi._line._mode)"></a><span style="font-style: italic">multi-line mode</span>, where <span class="RktInBG"><span class="hspace"></span><span class="RktIn">.</span><span class="hspace"></span></span> does not match a newline
character, <span class="RktInBG"><span class="hspace"></span><span class="RktIn">^</span><span class="hspace"></span></span> can match just after a newline, and <span class="RktInBG"><span class="hspace"></span><span class="RktIn">$</span><span class="hspace"></span></span>
can match just before a newline.</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktVal">#rx"."</span><span class="hspace"> </span><span class="RktVal">"\na\n"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'("\n")</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktVal">#rx"(?m:.)"</span><span class="hspace"> </span><span class="RktVal">"\na\n"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'("a")</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktVal">#rx"^A plan$"</span><span class="hspace"> </span><span class="RktVal">"A man\nA plan\nA canal"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">#f</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktVal">#rx"(?m:^A plan$)"</span><span class="hspace"> </span><span class="RktVal">"A man\nA plan\nA canal"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'("A plan")</span></p></td></tr></table></blockquote><p>You can put more than one modifier in the cloister:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktVal">#rx"(?mi:^A Plan$)"</span><span class="hspace"> </span><span class="RktVal">"a man\na plan\na canal"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'("a plan")</span></p></td></tr></table></blockquote><p>A minus sign before a modifier inverts its meaning. Thus, you can use
<span class="RktInBG"><span class="hspace"></span><span class="RktIn">-i</span><span class="hspace"></span></span> in a <a name="(tech._subcluster)"></a><span style="font-style: italic">subcluster</span> to overturn the
case-insensitivities caused by an enclosing cluster.</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktVal">#rx"(?i:the (?-i:TeX)book)"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"The TeXbook"</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'("The TeXbook")</span></p></td></tr></table></blockquote><p>The above regexp will allow any casing for <span class="RktInBG"><span class="hspace"></span><span class="RktIn">the</span><span class="hspace"></span></span> and
<span class="RktInBG"><span class="hspace"></span><span class="RktIn">book</span><span class="hspace"></span></span>, but it insists that <span class="RktInBG"><span class="hspace"></span><span class="RktIn">TeX</span><span class="hspace"></span></span> not be differently
cased.</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, "6.1", "../");" 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("6.1");">top</a></span><span class="navright"> <a href="regexp-quant.html" title="backward to "9.5 Quantifiers"" data-pltdoc="x">← prev</a> <a href="regexp.html" title="up to "9 Regular Expressions"" data-pltdoc="x">up</a> <a href="regexp-alternation.html" title="forward to "9.7 Alternation"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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