Here's code that will convert any (properly ordered) roman numeral to a number. It uses a hash, which is something I'm pretty excited about since I don't feel like I totally understand hashes.

roman_to_integer.rb

def roman_to_integer string
	string.upcase!
  raise ArgumentError if /[MDCLXVI]*/.match(string).to_s != string
	letter_values = {'M' => 1000,
			 'D' => 500,
			 'C' => 100,
			 'L' => 50,
			 'X' => 10,
			 'V' => 5,
			 'I' => 1}
	sum = 0
	prev_val = 0
	numeral_array = string.split('')
	while true
		if numeral_array.empty?
			return sum
		end
		current_let = numeral_array.pop
		if letter_values[current_let] < prev_val
			sum -= letter_values[current_let]
		else
			sum += letter_values[current_let]
		end
		prev_val = letter_values[current_let]
	end
end

Revision two now is not case-sensitive for input