simple arithmetic parser

simple_arithmetic_parser.rb

# -*- encoding : utf-8 -*-
require 'debugger'

cal = lambda do |exp, stack=[]|
  return exp if exp =~/^\d+$/

  exp.scan(/((?:\d+)|[\*\+\/-]|(?:\((?:(?:\d)|[\*\+\/-])+\)))/) do |match|
    word = match.first.to_s
    if word =~ /^(\d+)$/
      stack.unshift $1
    elsif word =~ /^(\(\d+[\*\+\/-]\d+\))$/
      return eval(exp)
    elsif word =~ /^([\*\+\/-]{1})$/
      stack.unshift $1
    else
      stack.unshift stack.pop.to_i.__send__(stack.pop, cal.call(word, stack).to_i)
    end
  end
end
puts cal.call("((9+19)-(31*2))+2")