1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>The Leftmost Longest Rule</title> 5<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../index.html" title="Boost.Regex 5.1.4"> 8<link rel="up" href="../syntax.html" title="Regular Expression Syntax"> 9<link rel="prev" href="collating_names/named_unicode.html" title="Named Unicode Characters"> 10<link rel="next" href="../format.html" title="Search and Replace Format String Syntax"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="collating_names/named_unicode.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../syntax.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../format.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h3 class="title"> 27<a name="boost_regex.syntax.leftmost_longest_rule"></a><a class="link" href="leftmost_longest_rule.html" title="The Leftmost Longest Rule">The Leftmost 28 Longest Rule</a> 29</h3></div></div></div> 30<p> 31 Often there is more than one way of matching a regular expression at a particular 32 location, for POSIX basic and extended regular expressions, the "best" 33 match is determined as follows: 34 </p> 35<div class="orderedlist"><ol class="orderedlist" type="1"> 36<li class="listitem"> 37 Find the leftmost match, if there is only one match possible at this 38 location then return it. 39 </li> 40<li class="listitem"> 41 Find the longest of the possible matches, along with any ties. If there 42 is only one such possible match then return it. 43 </li> 44<li class="listitem"> 45 If there are no marked sub-expressions, then all the remaining alternatives 46 are indistinguishable; return the first of these found. 47 </li> 48<li class="listitem"> 49 Find the match which has matched the first sub-expression in the leftmost 50 position, along with any ties. If there is only on such match possible 51 then return it. 52 </li> 53<li class="listitem"> 54 Find the match which has the longest match for the first sub-expression, 55 along with any ties. If there is only one such match then return it. 56 </li> 57<li class="listitem"> 58 Repeat steps 4 and 5 for each additional marked sub-expression. 59 </li> 60<li class="listitem"> 61 If there is still more than one possible match remaining, then they are 62 indistinguishable; return the first one found. 63 </li> 64</ol></div> 65</div> 66<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 67<td align="left"></td> 68<td align="right"><div class="copyright-footer">Copyright © 1998-2013 John Maddock<p> 69 Distributed under the Boost Software License, Version 1.0. (See accompanying 70 file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) 71 </p> 72</div></td> 73</tr></table> 74<hr> 75<div class="spirit-nav"> 76<a accesskey="p" href="collating_names/named_unicode.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../syntax.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../format.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 77</div> 78</body> 79</html> 80
