Second version of arabic to roman numerals converter

arabic.hs

arabic :: String -> Int
arabic "" = 0
arabic (x:y:xs)
  | x == 'C', y == 'M' = 900 + arabic xs
  | x == 'C', y == 'D' = 400 + arabic xs
  | x == 'I', y == 'X' = 9 + arabic xs
  | x == 'I', y == 'V' = 4 + arabic xs
arabic (x:xs)
  | x == 'M' = 1000 + arabic xs
  | x == 'D' = 500 + arabic xs
  | x == 'C' = 100 + arabic xs
  | x == 'X' = 10 + arabic xs
  | x == 'V' = 5 + arabic xs
  | x == 'I' = 1 + arabic xs
  | otherwise = 0 + arabic xs

First version doesn't work for combinations like CM, CD, IX, IV.

Second version works for numbers like MCMXXV (1925), but unfortunately, also for VIVI, which is invalid, but becomes V (5) + IV (4) + I (1)

Removing repetitions so that code is cleaner than before, but I'm sure the final part could be replaced with a tuple list.