1[/ 2 Copyright 2006-2007 John Maddock. 3 Distributed under the Boost Software License, Version 1.0. 4 (See accompanying file LICENSE_1_0.txt or copy at 5 http://www.boost.org/LICENSE_1_0.txt). 6] 7 8 9[section:leftmost_longest_rule The Leftmost Longest Rule] 10 11Often there is more than one way of matching a regular expression at a 12particular location, for POSIX basic and extended regular expressions, 13the "best" match is determined as follows: 14 15# Find the leftmost match, if there is only one match possible at 16this location then return it. 17# Find the longest of the possible matches, along with any ties. 18If there is only one such possible match then return it. 19# If there are no marked sub-expressions, then all the remaining 20alternatives are indistinguishable; return the first of these found. 21# Find the match which has matched the first sub-expression in the 22leftmost position, along with any ties. If there is only on such 23match possible then return it. 24# Find the match which has the longest match for the first sub-expression, 25along with any ties. If there is only one such match then return it. 26# Repeat steps 4 and 5 for each additional marked sub-expression. 27# If there is still more than one possible match remaining, then they are 28indistinguishable; return the first one found. 29 30[endsect] 31 32 33