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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>9.3 Basic Assertions</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" 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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="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="" class="tocviewselflink" 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, "6.1", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.1");">top</a></span><span class="navright"> <a href="regexp-match.html" title="backward to "9.2 Matching Regexp Patterns"" data-pltdoc="x">← prev</a> <a href="regexp.html" title="up to "9 Regular Expressions"" data-pltdoc="x">up</a> <a href="regexp-chars.html" title="forward to "9.4 Characters and Character Classes"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-part-tag=""regexp-assert"">9.3<tt> </tt><a name="(part._regexp-assert)"></a>Basic Assertions</h4><p>The <a name="(tech._assertion)"></a><span style="font-style: italic">assertions</span> <span class="RktInBG"><span class="hspace"></span><span class="RktIn">^</span><span class="hspace"></span></span> and <span class="RktInBG"><span class="hspace"></span><span class="RktIn">$</span><span class="hspace"></span></span> identify the
beginning and the end of the text string, respectively. They ensure
that their adjoining regexps match at one or other end of the text
string:</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-positions%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match-positions</a></span><span class="hspace"> </span><span class="RktVal">#rx"^contact"</span><span class="hspace"> </span><span class="RktVal">"first contact"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">#f</span></p></td></tr></table></blockquote><p>The <a href="regexp.html#%28tech._regexp%29" class="techoutside" data-pltdoc="x"><span class="techinside">regexp</span></a> above fails to match because <span class="RktInBG"><span class="hspace"></span><span class="RktIn">contact</span><span class="hspace"></span></span> does
not occur at the beginning of the text string. In</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-positions%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match-positions</a></span><span class="hspace"> </span><span class="RktVal">#rx"laugh$"</span><span class="hspace"> </span><span class="RktVal">"laugh laugh laugh laugh"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'((18 . 23))</span></p></td></tr></table></blockquote><p>the regexp matches the <span style="font-style: italic">last</span> <span class="RktInBG"><span class="hspace"></span><span class="RktIn">laugh</span><span class="hspace"></span></span>.</p><p>The metasequence <span class="RktInBG"><span class="hspace"></span><span class="RktIn">\b</span><span class="hspace"></span></span> asserts that a word boundary exists, but
this metasequence works only with <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#px</span><span class="hspace"></span></span> syntax. In</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-positions%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match-positions</a></span><span class="hspace"> </span><span class="RktVal">#px"yack\\b"</span><span class="hspace"> </span><span class="RktVal">"yackety yack"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'((8 . 12))</span></p></td></tr></table></blockquote><p>the <span class="RktInBG"><span class="hspace"></span><span class="RktIn">yack</span><span class="hspace"></span></span> in <span class="RktInBG"><span class="hspace"></span><span class="RktIn">yackety</span><span class="hspace"></span></span> doesn’t end at a word boundary
so it isn’t matched. The second <span class="RktInBG"><span class="hspace"></span><span class="RktIn">yack</span><span class="hspace"></span></span> does and is.</p><p>The metasequence <span class="RktInBG"><span class="hspace"></span><span class="RktIn">\B</span><span class="hspace"></span></span> (also <span class="RktInBG"><span class="hspace"></span><span class="RktIn">#px</span><span class="hspace"></span></span> only) has the
opposite effect to <span class="RktInBG"><span class="hspace"></span><span class="RktIn">\b</span><span class="hspace"></span></span>; it asserts that a word boundary does
not exist. In</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-positions%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">regexp-match-positions</a></span><span class="hspace"> </span><span class="RktVal">#px"an\\B"</span><span class="hspace"> </span><span class="RktVal">"an analysis"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'((3 . 5))</span></p></td></tr></table></blockquote><p>the <span class="RktInBG"><span class="hspace"></span><span class="RktIn">an</span><span class="hspace"></span></span> that doesn’t end in a word boundary is matched.</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-match.html" title="backward to "9.2 Matching Regexp Patterns"" data-pltdoc="x">← prev</a> <a href="regexp.html" title="up to "9 Regular Expressions"" data-pltdoc="x">up</a> <a href="regexp-chars.html" title="forward to "9.4 Characters and Character Classes"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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