Haskell subRegex with transformer, similar to gsub in ruby and other languages.

subregex.hs

import Text.Regex (mkRegex, matchRegexAll, Regex)

------------------------------------------------------------------------------
{- | Taken from the <http://pleac.sourceforge.net/pleac_haskell/strings.html>

     'subRegex' allows only a fixed 'String'
     This custom version takes a ('String' -> 'String') function to compute
     a result.

     The function is applied to the all matched results, which are found
     recursively.
-}
subRegexF :: Regex -> (String -> String) -> String -> String
subRegexF regex func str =
  case matchRegexAll regex str of
    Nothing -> str
    Just (before, matched, after, _) ->
      before ++ func matched ++ (subRegexF regex func after)
------------------------------------------------------------------------------