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</div><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-source-pkg="racket-doc" x-part-tag=""regexp-alternation"">9.7<tt> </tt><a name="(part._regexp-alternation)"></a>Alternation</h4><p>You can specify a list of <span style="font-style: italic">alternate</span> <a href="regexp-clusters.html#%28tech._subpattern%29" class="techoutside" data-pltdoc="x"><span class="techinside">subpatterns</span></a> by
separating them by <span class="RktInBG"><span class="hspace"></span><span class="RktIn">|</span><span class="hspace"></span></span>. The <span class="RktInBG"><span class="hspace"></span><span class="RktIn">|</span><span class="hspace"></span></span> separates
<a href="regexp-clusters.html#%28tech._subpattern%29" class="techoutside" data-pltdoc="x"><span class="techinside">subpatterns</span></a> in the nearest enclosing cluster (or in the entire
pattern string if there are no enclosing parens).</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%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktVal">#rx"f(ee|i|o|um)"</span><span class="hspace"> </span><span class="RktVal">"a small, final fee"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'("fi" "i")</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="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-replace%252A%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">regexp-replace*</a></span><span class="hspace"> </span><span class="RktVal">#rx"([yi])s(e[sdr]?|ing|ation)"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </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=strings.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._string-append%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">string-append</a></span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"analyse an energising organisation"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">" pulsing with noisy organisms"</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"\\1z\\2"</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">"analyze an energizing organization pulsing with noisy organisms"</span></p></td></tr></table></blockquote><p>Note again that if you wish to use clustering merely to specify a list
of alternate subpatterns but do not want the submatch, 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>.</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%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktVal">#rx"f(?:ee|i|o|um)"</span><span class="hspace"> </span><span class="RktVal">"fun for all"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">'("fo")</span></p></td></tr></table></blockquote><p>An important thing to note about alternation is that the leftmost
matching alternate is picked regardless of its length. Thus, if one
of the alternates is a prefix of a later alternate, the latter may not
have a chance to 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/docs/6.7/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktVal">#rx"call|call-with-current-continuation"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"call-with-current-continuation"</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'("call")</span></p></td></tr></table></blockquote><p>To allow the longer alternate to have a shot at matching, place it
before the shorter one:</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/docs/6.7/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span><span class="hspace"> </span><span class="RktVal">#rx"call-with-current-continuation|call"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"call-with-current-continuation"</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'("call-with-current-continuation")</span></p></td></tr></table></blockquote><p>In any case, an overall match for the entire regexp is always
preferred to an overall non-match. In the following, the longer
alternate still wins, because its preferred shorter prefix fails to
yield an overall 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/docs/6.7/html/local-redirect/index.html?doc=reference&rel=regexp.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._regexp-match%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">regexp-match</a></span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">#rx"(?:call|call-with-current-continuation) constrained"</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">"call-with-current-continuation constrained"</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'("call-with-current-continuation constrained")</span></p></td></tr></table></blockquote><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-clusters.html" title="backward to "9.6 Clusters"" data-pltdoc="x">← prev</a> <a href="regexp.html" title="up to "9 Regular Expressions"" data-pltdoc="x">up</a> <a href="Backtracking.html" title="forward to "9.8 Backtracking"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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