This library allows you to create types that are guaranteed to contain a string that matches a given regular expression which is expressed at the type level.
newtype User = User (Matched String "^[a-zA-Z0-9]{4,15}$")
parseUser :: String -> Either (RegexError String) User
parseUser = fmap User . parseMatchedEither
prettyUser :: User -> String
prettyUser (User m) = asString m
main :: IO ()
main = do
l <- getLine
case parseUser l of
Right user -> putStrLn $ "Hello, " ++ prettyUser user ++ "!"
Left error -> putStrLn $ "Could not parse username: " ++ prettyRegexError error./prog
bad
Could not parse username: The input "bad" did not match the pattern ^[a-zA-Z0-9]{4,15}$
./prog
good
Hello, good!