File: m4.info,  Node: Regexp,  Next: Substr,  Prev: Index macro,  Up: Text handling

11.3 Searching for regular expressions
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Searching for regular expressions is done with the builtin ‘regexp’:

 -- Builtin: regexp(STRING, REGEXP, [REPLACEMENT])
     Searches for REGEXP in STRING.  The syntax for regular expressions
     is closest to older GNU Emacs (grouping via ‘\(’ and ‘\)’,
     alternation via ‘\|’, one-or-more matching via ‘+’, zero-or-one
     matching via ‘?’), although for backwards compatibility reasons, M4
     1.4.x does not enable intervals (that is, both ‘{’ and ‘\{’ match a
     literal ‘{’), nor character classes (that is, ‘[[:alpha:]]’ matches
     a sequence of one of six bytes followed by a literal close bracket,
     rather than all letters, the same as ‘[[ahlp:]’ followed by ‘]’).
     *Note Syntax of Regular Expressions: (emacs)Regexps.  Support for
     intervals, character classes, non-greedy matches, or even
     alternative regular expression syntax (such as POSIX Basic or
     Extended Regular Expressions) are likely to be added in a future
     version of GNU M4.

     If REPLACEMENT is omitted, ‘regexp’ expands to the index of the
     first match of REGEXP in STRING.  If REGEXP does not match anywhere
     in STRING, it expands to -1.

     If REPLACEMENT is supplied, and there was a match, ‘regexp’ changes
     the expansion to this argument, with ‘\N’ substituted by the text
     matched by the Nth parenthesized sub-expression of REGEXP, up to
     nine sub-expressions.  The escape ‘\&’ is replaced by the text of
     the entire regular expression matched.  For all other characters,
     ‘\’ treats the next character literally.  A warning is issued if
     there were fewer sub-expressions than the ‘\N’ requested, or if
     there is a trailing ‘\’.  If there was no match, ‘regexp’ expands
     to the empty string.

     The macro ‘regexp’ is recognized only with parameters.

     regexp(`GNUs not Unix', `\<[a-z]\w+')
     ⇒5
     regexp(`GNUs not Unix', `\