四則演算のパーサー（Ruby）

operations_parser.rb

def parse(text)
  splited = text.scan(/[0-9\.]+|\+|\-|\*|\/|\(|\)|=/)
  output = []
  stack = []
  a = nil
  
  until (token = splited.shift) == "="
    case token
    when "(" then stack << token
    when ")"
      output << a until (a = stack.pop) == "("
    when "*", "/"
      loop do
        a = stack.last
        break unless %w(* /).include?(a)
        output << stack.pop
      end
      stack << token
    when "+", "-"
      loop do
        a = stack.last
        break unless %w(+ - * /).include?(a)
        output << stack.pop
      end
      stack << token
    else
      output << token
    end
  end
  
  output << a while (a = stack.pop)
  return output
end

def calculate(parsed)
  stack = []
  while (x = parsed.shift)
    if %w(+ - * /).include?(x)
      a, b = stack.pop, stack.pop
      stack << eval("#{b} #{x} #{a}")
    else
      stack << x
    end
  end
  stack.first
end