regex-applicative is a parsing combinator library for Haskell based on regular expressions.

import Text.Regex.Applicative

data Protocol = HTTP | FTP deriving Show

protocol :: RE Char Protocol
protocol = HTTP <$ string "http" <|> FTP <$ string "ftp"

type Host = String
type Location = String
data URL = URL Protocol Host Location deriving Show

host :: RE Char Host
host = many $ psym $ (/= '/')

url :: RE Char URL
url = URL <$> protocol <* string "://" <*> host <* sym '/' <*> many anySym

main = print $ "http://stackoverflow.com/questions" =~ url

See the API reference.

For common tasks, this package is several times slower than monadic parser combinator libraries like parsec. However, this library has a roughly linear complexity, whereas monadic parser combinators have exponential worst-time complexity (see here).

Some tips to make your regex run faster:

1. If you don't care about the result of the whole regex or its part, only whether it matches or not, mark it with void or <$. Recognition is faster than parsing.

2. If you apply the same regex to multiple strings, partially apply it like so:

   let matcher = match my_regex
   in  map matcher my_strings
   

   This way the compiled regex is stored in the matcher value and shared among the strings.

Only GHC versions >= 8.0 are supported, although older versions may work too.