Roman numerals conversion to decimal in Scala

RomanToDecimal.scala

def numerals: List[char]): int = in match {
    case 'i' :: 'v' :: xs => 4    + numerals(xs)
    case 'i' :: 'X' :: xs => 9    + numerals(xs)
    case 'i' :: xs        => 1    + numerals(xs)
    case 'v' :: xs        => 5    + numerals(xs)
    case 'X' :: 'L' :: xs => 40   + numerals(xs)
    case 'X' :: 'c' :: xs => 90   + numerals(xs)
    case 'X' :: xs        => 10   + numerals(xs)
    case 'L' :: xs        => 50   + numerals(xs)
    case 'c' :: 'd' :: xs => 400  + numerals(xs)
    case 'c' :: 'm' :: xs => 900  + numerals(xs)
    case 'c' :: xs        => 100  + numerals(xs)
    case 'd' :: xs        => 500  + numerals(xs)
    case 'm' :: xs        => 1000 + numerals(xs)
    case _                => 0
}