Sample Eval

def calc_rev_pol(expr)
  return 0 if expr.empty?
  ary = expr.scan(/\d+\.?\d?|\D/).delete_if{|token| token == ' '}
  stack = []
  ary.each do |token|
    if %w(+ - / *).include?(token)
      val2 = stack.pop.to_f
      val1 = stack.pop.to_f
      stack.push(val1.send(token, val2))
    else
      stack.push(token)
    end
  end
  stack.pop.to_f
end