To/From Roman numerals

roman.rb

class Fixnum
  def to_roman
    return '' if self == 0
    roman, arabic = { 
      M: 1000,
      CM: 900,
      D: 500,
      CD: 400,
      C: 100,
      XC: 90, 
      L: 50, 
      XL: 40, 
      X: 10, 
      IX: 9,
      V: 5,
      IV: 4,
      I: 1
    }.detect {|(_,a)| a <= self}
    roman.to_s + (self - arabic).to_roman
  end 
end

class String
  def from_roman
    return 0 if self == ''
    roman, arabic = { 
      M: 1000,
      CM: 900,
      D: 500,
      CD: 400,
      C: 100,
      XC: 90, 
      L: 50, 
      XL: 40, 
      X: 10, 
      IX: 9,
      V: 5,
      IV: 4,
      I: 1
    }.detect {|(r,_)| self =~ /^#{r}/ }
    arabic + self.sub(/^#{roman}/,'').from_roman
  end 
end