import Text.Parsec
import Text.Parsec.String

domain :: Parser String
domain = do
  string "https" <|> string "http"
  string "://"
  many1 (noneOf "/")

λ> parse domain "" "https://www.google.com/"
Right "www.google.com"

λ> parse domain "" "http://www.google.com/"
Left (line 1, column 1):
unexpected ":"
expecting "https"