(m4.info)Regexp


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11.3 Searching for regular expressions
======================================

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.  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


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