Info Catalog m4: Index macro m4: Text handling m4: Substr

m4: Regexp

 
 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. ⇒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
 

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