Convert roman numerals into arabic numerals in place

string.rb

class String

  def to_arabic
    result = 0
    str = self
    roman_mapping.each_value do |roman|
      while str.start_with?(roman)
        result += roman_mapping.invert[roman]
        str = str.slice(roman.length, str.length)
      end
    end
    result.zero? ? '' : result.to_s
  end

  def convert_roman_in_place
    gsub(/\bM{0,4}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})\b/m) { |s| s.to_arabic }
  end
 
  private
 
  def roman_mapping
    {
      1000 => "M",
      900 => "CM",
      500 => "D",
      400 => "CD",
      100 => "C",
      90 => "XC",
      50 => "L",
      40 => "XL",
      10 => "X",
      9 => "IX",
      5 => "V",
      4 => "IV",
      1 => "I"
    }
  end
 
end