Question

Do any of the Haskell PCRE libraries provide a function to escape regex metacharacters in a string? I.e. a function to take a string like “[$100]” and turn it into “\[\$100\]”.

I’m looking for the equivalent of Python’s re.escape, which I can’t seem to find in regex-pcre.

Answer 1

I’m not aware of such a function in any of
the PCRE libraries, but depending on what
you are trying to accomplish you could use
PCRE quoting:

{-# LANGUAGE OverloadedStrings #-}

import qualified Data.ByteString.Char8 as B
import Text.Regex.PCRE


quotePCRE bs = B.concat [ "\\Q" , bs , "\\E" ]

-- Of course, this won't work if the
-- string to be quoted contains `\E` ,
-- but that would be much eaiser to fix
-- than writing a function taking into
-- account all the necessary escaping.

literal = "^[$100]$"

quoted = quotePCRE literal

main :: IO ()
main = do B.putStr "literal: " >> B.putStrLn literal

          -- literal: ^[$100]$

          B.putStr "quoted: "  >> B.putStrLn quoted

          -- quoted: \Q^[$100]$\E

          putStrLn "literal =~ literal :: Bool"
          print ( literal =~ literal :: Bool )

          -- literal =~ literal :: Bool
          -- False

          putStrLn "literal =~ quoted :: Bool"
          print ( literal =~ quoted :: Bool )

          -- literal =~ quoted :: Bool
          -- True