<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>The Leftmost Longest Rule</title> <link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.74.0"> <link rel="home" href="../../index.html" title="Boost.Regex"> <link rel="up" href="../syntax.html" title="Regular Expression Syntax"> <link rel="prev" href="collating_names/named_unicode.html" title="Named Unicode Characters"> <link rel="next" href="../format.html" title="Search and Replace Format String Syntax"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> <td align="center"><a href="../../../../../../index.html">Home</a></td> <td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <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> </div> <div class="section" lang="en"> <div class="titlepage"><div><div><h3 class="title"> <a name="boost_regex.syntax.leftmost_longest_rule"></a><a class="link" href="leftmost_longest_rule.html" title="The Leftmost Longest Rule"> The Leftmost Longest Rule</a> </h3></div></div></div> <p> Often there is more than one way of matching a regular expression at a particular location, for POSIX basic and extended regular expressions, the "best" match is determined as follows: </p> <div class="orderedlist"><ol type="1"> <li> Find the leftmost match, if there is only one match possible at this location then return it. </li> <li> Find the longest of the possible matches, along with any ties. If there is only one such possible match then return it. </li> <li> If there are no marked sub-expressions, then all the remaining alternatives are indistinguishable; return the first of these found. </li> <li> Find the match which has matched the first sub-expression in the leftmost position, along with any ties. If there is only on such match possible then return it. </li> <li> Find the match which has the longest match for the first sub-expression, along with any ties. If there is only one such match then return it. </li> <li> Repeat steps 4 and 5 for each additional marked sub-expression. </li> <li> If there is still more than one possible match remaining, then they are indistinguishable; return the first one found. </li> </ol></div> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"></td> <td align="right"><div class="copyright-footer">Copyright © 1998 -2007 John Maddock<p> Distributed under the Boost Software License, Version 1.0. (See accompanying 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>) </p> </div></td> </tr></table> <hr> <div class="spirit-nav"> <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> </div> </body> </html>