(m4.info.gz) Patsubst

 
 Substituting text by regular expression
 =======================================
 
    Global substitution in a string is done by `patsubst':
 
      patsubst(STRING, REGEXP, opt REPLACEMENT)
 
 which searches STRING for matches of REGEXP, and substitutes
 REPLACEMENT for each match.  The syntax for regular expressions is the
 same as in GNU Emacs.
 
    The parts of STRING that are not covered by any match of REGEXP are
 copied to the expansion.  Whenever a match is found, the search
 proceeds from the end of the match, so a character from STRING will
 never be substituted twice.  If REGEXP matches a string of zero length,
 the start position for the search is incremented, to avoid infinite
 loops.
 
    When a replacement is to be made, REPLACEMENT is inserted into the
 expansion, with `\N' substituted by the text matched by the Nth
 parenthesized sub-expression of REGEXP, `\&' being the text the entire
 regular expression matched.
 
    The REPLACEMENT argument can be omitted, in which case the text
 matched by REGEXP is deleted.
 
      patsubst(`GNUs not Unix', `^', `OBS: ')
      =>OBS: GNUs not Unix
      patsubst(`GNUs not Unix', `\<', `OBS: ')
      =>OBS: GNUs OBS: not OBS: Unix
      patsubst(`GNUs not Unix', `\w*', `(\&)')
      =>(GNUs)() (not)() (Unix)
      patsubst(`GNUs not Unix', `\w+', `(\&)')
      =>(GNUs) (not) (Unix)
      patsubst(`GNUs not Unix', `[A-Z][a-z]+')
      =>GN not
 
    Here is a slightly more realistic example, which capitalizes
 individual word or whole sentences, by substituting calls of the macros
 `upcase' and `downcase' into the strings.
 
      define(`upcase', `translit(`$*', `a-z', `A-Z')')dnl
      define(`downcase', `translit(`$*', `A-Z', `a-z')')dnl
      define(`capitalize1',
           `regexp(`$1', `^\(\w\)\(\w*\)', `upcase(`\1')`'downcase(`\2')')')dnl
      define(`capitalize',
           `patsubst(`$1', `\w+', `capitalize1(`\&')')')dnl
      capitalize(`GNUs not Unix')
      =>Gnus Not Unix
 
    The builtin macro `patsubst' is recognized only when given arguments.
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