Ruby Dice Roller

dice.rb

def roll expression
	return [0] if (/^(?<count>\d+)d(?<sides>\d+)(?:s(?<drop>\d+))?$/i =~ expression ).nil?
	
	Array.new( count.to_i, nil ).
	      map { (Random.rand sides.to_i) + 1 }.
	      sort.reverse.
	      take( count.to_i - drop.to_i )
end

expr = "(#{ARGV.join ' '})"; last = ''
until last == expr
	last = expr
	expr = expr.sub(/(\d+d\d+(?:s\d+)?)/i) { |expr| roll(expr).inject(:+).to_s }.
	            sub(/(\d+)([\+\-\*\/])(\d+)/) { |m| m[0].to_i.send m[1].to_s, m[2].to_i }.
	            sub(/\(\d+\)/) { |n| n[1..-2] }
end

puts expr