class RPNCalculator
  def evaluate(rpn_expr)
    begin
      count = 0
      result = rpn_expr.split(' ').each_with_object([]) do |item,arr|
        item.match(/(\A-?\d+\z)|(\A[\+\-\*]\z)/) do
          if $1
            count += 1
            arr << item.to_i
          else
            count -= 1
            raise ArgumentError.new("not enough digits!") if count < 1
            arr[-2..-1] = ( arr[-2].send(item, arr[-1]) )
          end
        end or raise ArgumentError.new("malformed RPN expression!")
      end
      count == 1 && result.first or raise ArgumentError.new("too many digits!")
    rescue ArgumentError => e
      e.message
    end
  end
end