regex-to-plain-string

import Control.Monad
import Data.Maybe

regexChars = "^$(|)[]{}.*"
regexCharsLast = "$)]}.*"

toPlainString' :: String -> String -> Maybe String
toPlainString' s []         = Just s
toPlainString' s (r:[])     = if r `elem` regexCharsLast
                              then Nothing
                              else Just (r:s)
toPlainString' s (r1:r2:rs) = if r1 `elem` regexChars
                              then Nothing
                              else if r1 == '\\' && r2 `elem` ('\\':regexChars)
                                   then toPlainString' (r2:s) rs
                                   else toPlainString' (r1:s) (r2:rs)
toPlainString str = liftM reverse $ toPlainString' [] str