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Boost.Regex

The "Leftmost Longest" Rule


Often there is more that 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:

  1. Find the leftmost match, if there is only one match possible at this location then return it.
  2. Find the longest of the possible matches, along with any ties.  If there is only one such possible match then return it.
  3. If there are no marked sub-expressions, then all the remaining alternatives are indistinguishable; return the first of these found.
  4. 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.
  5. 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.
  6. Repeat steps 3 and 4 for each additional marked sub-expression.
  7. If there is still more than one possible match remaining, then they are indistinguishable; return the first one found.


Revised 16 Dec 2004

Copyright John Maddock 1998- 2003

Use, modification and distribution are subject to the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)