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<title>The Leftmost Longest Rule</title>
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<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>
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<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 class="orderedlist" type="1">
<li class="listitem">
            Find the leftmost match, if there is only one match possible at this
            location then return it.
          </li>
<li class="listitem">
            Find the longest of the possible matches, along with any ties. If there
            is only one such possible match then return it.
          </li>
<li class="listitem">
            If there are no marked sub-expressions, then all the remaining alternatives
            are indistinguishable; return the first of these found.
          </li>
<li class="listitem">
            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 class="listitem">
            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 class="listitem">
            Repeat steps 4 and 5 for each additional marked sub-expression.
          </li>
<li class="listitem">
            If there is still more than one possible match remaining, then they are
            indistinguishable; return the first one found.
          </li>
</ol></div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 1998-2013 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>)
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