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(elisp)POSIX Regexps

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POSIX Regular Expression Searching
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   The usual regular expression functions do backtracking when necessary
to handle the `\|' and repetition constructs, but they continue this
only until they find _some_ match.  Then they succeed and report the
first match found.

   This section describes alternative search functions which perform the
full backtracking specified by the POSIX standard for regular expression
matching.  They continue backtracking until they have tried all
possibilities and found all matches, so they can report the longest
match, as required by POSIX.  This is much slower, so use these
functions only when you really need the longest match.

 - Function: posix-search-forward regexp &optional limit noerror repeat
     This is like `re-search-forward' except that it performs the full
     backtracking specified by the POSIX standard for regular expression
     matching.

 - Function: posix-search-backward regexp &optional limit noerror repeat
     This is like `re-search-backward' except that it performs the full
     backtracking specified by the POSIX standard for regular expression
     matching.

 - Function: posix-looking-at regexp
     This is like `looking-at' except that it performs the full
     backtracking specified by the POSIX standard for regular expression
     matching.

 - Function: posix-string-match regexp string &optional start
     This is like `string-match' except that it performs the full
     backtracking specified by the POSIX standard for regular expression
     matching.