Haskell: Text.Regex.PCRE.ByteString

I'm pausing my study of existing regex libraries; I'm somewhat burnt out by the mess that's out there. For what it's worth, I've figured out a mid-level API use-case for PCRE and ByteStrings, which are processed faster than Strings. I haven't been able to get the equivalent use-case to work for the TDFA package.

{-# LANGUAGE OverloadedStrings #-} 

import Text.Regex.PCRE.ByteString
import Data.ByteString.Char8
  -- For instances of the IsString type class. FTS.
import Data.Array (Array)
  -- Not really needed, unless you want to make certain type signatures
  -- explicit.

-- Gets the before / match / after ByteStrings.
_regexec
  :: IO
       (Either
          WrapError
          (Maybe (ByteString, ByteString, ByteString, [ByteString])))
_regexec = do
  regex <- _regex
  regexec regex "abCdefgbChijklmbCno" 

-- Gets the first match offset and length.
_execute
  :: IO
       (Either
          WrapError (Maybe (Array Int (MatchOffset, MatchLength))))
_execute = do
  regex <- _regex
  execute regex "abCdefgbChijklmbCno"

_regex :: IO Regex
_regex = do
  compiled <- compile compMultiline execBlank "b.(.*?)b."
    -- These compX and execY settings correspond to defaults defined in
    -- Text.Regex.PCRE.Wrap
  case compiled of
    Right regex 
      -> return regex