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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><meta name="viewport" content="width=device-width, initial-scale=0.8"/><title>9.9 Looking Ahead and Behind</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">The Racket Guide</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="t
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<span class="emph">behind</span> to ensure that a subpattern does or does not occur.
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These “look around” assertions are specified by putting the
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subpattern checked for in a cluster whose leading characters are:
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<span class="RktInBG"><span class="hspace"></span><span class="RktIn">?=</span><span class="hspace"></span></span> (for positive lookahead), <span class="RktInBG"><span class="hspace"></span><span class="RktIn">?!</span><span class="hspace"></span></span> (negative
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lookahead), <span class="RktInBG"><span class="hspace"></span><span class="RktIn">?<=</span><span class="hspace"></span></span> (positive lookbehind), <span class="RktInBG"><span class="hspace"></span><span class="RktIn">?<!</span><span class="hspace"></span></span>
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(negative lookbehind). Note that the subpattern in the assertion does
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not generate a match in the final result; it merely allows or
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disallows the rest of the match.</p><h5 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-source-pkg="racket-doc" x-part-tag=""Lookahead"">9.9.1<tt> </tt><a name="(part._.Lookahead)"></a>Lookahead</h5><p>Positive lookahead with <span class="RktInBG"><span class="hspace"></span><span class="RktIn">?=</span><span class="hspace"></span></span> peeks ahead to ensure that
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its subpattern <span class="emph">could</span> match.</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="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match-positions%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">regexp-match-positions</a></span><span class="hspace"> </span><span class="RktVal">#rx"grey(?=hound)"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"i left my grey socks at the greyhound"</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'((28 . 32))</span></p></td></tr></table></blockquote><p>The regexp <span class="RktVal">#rx"grey(?=hound)"</span> matches <span class="RktInBG"><span class="hspace"></span><span class="RktIn">grey</span><span class="hspace"></span></span>, but
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<span class="emph">only</span> if it is followed by <span class="RktInBG"><span class="hspace"></span><span class="RktIn">hound</span><span class="hspace"></span></span>. Thus, the first
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<span class="RktInBG"><span class="hspace"></span><span class="RktIn">grey</span><span class="hspace"></span></span> in the text string is not matched.</p><p>Negative lookahead with <span class="RktInBG"><span class="hspace"></span><span class="RktIn">?!</span><span class="hspace"></span></span> peeks ahead to ensure that its
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subpattern <span class="emph">could not</span> possibly match.</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="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match-positions%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">regexp-match-positions</a></span><span class="hspace"> </span><span class="RktVal">#rx"grey(?!hound)"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"the gray greyhound ate the grey socks"</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'((27 . 31))</span></p></td></tr></table></blockquote><p>The regexp <span class="RktVal">#rx"grey(?!hound)"</span> matches <span class="RktInBG"><span class="hspace"></span><span class="RktIn">grey</span><span class="hspace"></span></span>, but
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only if it is <span class="emph">not</span> followed by <span class="RktInBG"><span class="hspace"></span><span class="RktIn">hound</span><span class="hspace"></span></span>. Thus the
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<span class="RktInBG"><span class="hspace"></span><span class="RktIn">grey</span><span class="hspace"></span></span> just before <span class="RktInBG"><span class="hspace"></span><span class="RktIn">socks</span><span class="hspace"></span></span> is matched.</p><h5 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-source-pkg="racket-doc" x-part-tag=""Lookbehind"">9.9.2<tt> </tt><a name="(part._.Lookbehind)"></a>Lookbehind</h5><p>Positive lookbehind with <span class="RktInBG"><span class="hspace"></span><span class="RktIn">?<=</span><span class="hspace"></span></span> checks that its subpattern
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<span class="emph">could</span> match immediately to the left of the current position in
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the text 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="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match-positions%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">regexp-match-positions</a></span><span class="hspace"> </span><span class="RktVal">#rx"(?<=grey)hound"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"the hound in the picture is not a greyhound"</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'((38 . 43))</span></p></td></tr></table></blockquote><p>The regexp <span class="RktVal">#rx"(?<=grey)hound"</span> matches <span class="RktInBG"><span class="hspace"></span><span class="RktIn">hound</span><span class="hspace"></span></span>, but
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only if it is preceded by <span class="RktInBG"><span class="hspace"></span><span class="RktIn">grey</span><span class="hspace"></span></span>.</p><p>Negative lookbehind with <span class="RktInBG"><span class="hspace"></span><span class="RktIn">?<!</span><span class="hspace"></span></span> checks that its subpattern
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could not possibly match immediately to the left.</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="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match-positions%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">regexp-match-positions</a></span><span class="hspace"> </span><span class="RktVal">#rx"(?<!grey)hound"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"the greyhound in the picture is not a hound"</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'((38 . 43))</span></p></td></tr></table></blockquote><p>The regexp <span class="RktVal">#rx"(?<!grey)hound"</span> matches <span class="RktInBG"><span class="hspace"></span><span class="RktIn">hound</span><span class="hspace"></span></span>, but
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only if it is <span class="emph">not</span> preceded by <span class="RktInBG"><span class="hspace"></span><span class="RktIn">grey</span><span class="hspace"></span></span>.</p><p>Lookaheads and lookbehinds can be convenient when they
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are not confusing.</p><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" id="searchbox" type="text" tabindex="1" placeholder="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "8.6", "../");"/></form> <a href="https://docs.racket-lang.org/index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("8.6");">top</a><span class="tocsettoggle"> <a href="javascript:void(0);" title="show/hide table of contents" onclick="TocsetToggle();">contents</a></span></span><span class="navright"> <a href="Backtracking.html" title="backward to "9.8 Backtracking"" data-pltdoc="x">← prev</a> <a href="regexp.html" title="up to "9 Regular Expressions"" data-pltdoc="x">up</a> <a href="An_Extended_Example.html" title="forward to "9.10 An Extended Example"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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