data State = LeftSide | LeftPeriod | AtSymbol | RightSide | RightPeriod | End

correctEmails pattern
    | isValid pattern = rawr LeftPeriod pattern
    | otherwise = 0
    where isValid str = all (\x -> elem x "abcde?@.") str

-- |

rawr :: State -> String -> Int

rawr End [] = 1
rawr _ [] = 0

rawr state ('?':xs) = sum [rawr state (y:xs) | y <- (allowedChars state)] + 5 * rawr state ('a':xs)

rawr LeftSide ('@':xs) = rawr AtSymbol xs
rawr LeftSide ('.':xs) = rawr LeftPeriod xs
rawr LeftSide ( _ :xs) = rawr LeftSide xs

rawr LeftPeriod ('@': xs) = 0
rawr LeftPeriod ('.': xs) = 0
rawr LeftPeriod ( _ : xs) = rawr LeftSide xs

rawr AtSymbol ('@':xs) = 0
rawr AtSymbol ('.':xs) = 0
rawr AtSymbol ( _ :xs) = rawr RightSide xs

rawr RightSide ('@':xs) = 0
rawr RightSide ('.':xs) = rawr RightPeriod xs
rawr RightSide ( _ :xs) = rawr RightSide xs

rawr RightPeriod ('@':xs) = 0
rawr RightPeriod ('.':xs) = 0
rawr RightPeriod ( _ :xs) = rawr End xs

rawr End ('@':xs) = 0
rawr End ('.':xs) = rawr RightPeriod xs
rawr End ( _ :xs) = rawr End xs

-- |

allowedChars :: State -> [Char]
allowedChars LeftSide = "@."
allowedChars RightSide = "."
allowedChars End = "."
allowedChars _ = []