try to implement VBF.Compilers.Scanners.RegularExpression;

Regex.hs

{-# LANGUAGE OverloadedStrings #-}
module Regex where

import GHC.Exts(IsString(..))

data Regex
    = Empty
    | Concatention Regex Regex
    | Alternation Regex Regex
    | KleeneStar Regex
    | Symbol String

instance IsString Regex where
    fromString = Symbol

instance Show Regex where
    show Empty = ""
    show (Concatention r1 r2) = show r1 ++ show r2
    show (Alternation r1 r2) = show r1 ++ "|" ++ show r2
    show (KleeneStar r) = "[" ++ show r ++ "]*"
    show (Symbol s) = s

infix 5 <+>
(<+>) :: Regex -> Regex -> Regex
a <+> b  = Concatention a b

infix 4 <|>
(<|>) :: Regex -> Regex -> Regex
a <|> b  = Alternation a b

many :: Regex -> Regex
many     = KleeneStar

re :: Regex
re = many $ "a" <|> (many $ "b" <+> "a")