This is a really short implementation of a pattern matcher for a restricted class of regular expressions, namely those consisting of character literals and the special symbols . ^ & * where . matches any single character, ^ binds the pattern to the target string's beginning, $ binds the pattern to the target's ending, and * matches zero or more …

Hegex.hs

module Main (main) where

match :: String -> String -> Bool
match ('^':rest) text     = matchLocal rest text
match rex        (c:rest) = matchLocal rex (c:rest) || match rex rest
match _          _        = False

matchLocal :: String -> String -> Bool
matchLocal []            _         = True
matchLocal "$"           []        = True
matchLocal (c:'*':restR) (t:restT) = c == t && matchLocal restR (starSkip c (t:restT))
matchLocal (r:restR)     (t:restT) = (r == '.' || r == t) && matchLocal restR restT
matchLocal _             _         = False

starSkip :: Char -> String -> String
starSkip c (t:rest) = if c == t then starSkip c rest else t:rest
starSkip _ []       = []

main :: IO ()
main = do putStrLn "Enter regex:"
          regEx <- getLine
          putStrLn "Enter text:"
          text <- getLine
          putStrLn $ if match regEx text then "Match found." else "No match found."