(m4.info)Regexp

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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 the same as in GNU Emacs, which is similar to BRE, Basic Regular
     Expressions in POSIX. *Note Syntax of Regular Expressions:
     (emacs)Regexps.  Support for ERE, Extended Regular Expressions is
     not available, but will be added in GNU M4 2.0.

     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', `\<Q\w*')
     =>-1
     regexp(`GNUs not Unix', `\w\(\w+\)$', `*** \& *** \1 ***')
     =>*** Unix *** nix ***
     regexp(`GNUs not Unix', `\<Q\w*', `*** \& *** \1 ***')
     =>

   Here are some more examples on the handling of backslash:

     regexp(`abc', `\(b\)', `\\\10\a')
     =>\b0a
     regexp(`abc', `b', `\1\')
     error->m4:stdin:2: Warning: sub-expression 1 not present
     error->m4:stdin:2: Warning: trailing \ ignored in replacement
     =>
     regexp(`abc', `\(\(d\)?\)\(c\)', `\1\2\3\4\5\6')
     error->m4:stdin:3: Warning: sub-expression 4 not present
     error->m4:stdin:3: Warning: sub-expression 5 not present
     error->m4:stdin:3: Warning: sub-expression 6 not present
     =>c

   Omitting REGEXP evokes a warning, but still produces output; contrast
this with an empty REGEXP argument.

     regexp(`abc')
     error->m4:stdin:1: Warning: too few arguments to builtin `regexp'
     =>0
     regexp(`abc', `')
     =>0
     regexp(`abc', `', `\\def')
     =>\def