Ruby eval

def calc(expr)
  res = 0
  ary = expr.scan(/\d+|\D/).delete_if{|token| token == ' '}
  stack = []
  ary.each do |token|
    if token.is_operator?
     res += eval(stack.pop.to_i, token, stack.pop.to_i)
    p stack.push(res)
    else
      stack.push(token)
    end
  end
  stack.pop
end

def eval(num1, operator, num2)
  case operator
    when '+'
      num1 + num2
    when '-'
      num1 - num2
    when '*'
      num1 * num2
    when '/'
      num1.to_f / num2
  end
end


class String
  def is_operator?
    %w(+ - / *).include?(self)
  end
end