Ruby script to calculate a formula expressed with RPN.

rpn.rb

#! /usr/local/bin/ruby
#*********************************************************
# Ruby script to caculate a formula string by RPN.
#*********************************************************

class String
  RE_0  = Regexp.new('[=\s]+$')
  RE_1  = Regexp.new('\s+')
  RE_D  = Regexp.new('\d+')
  RE_PL = Regexp.new('\+')
  RE_MN = Regexp.new('\-')
  RE_PR = Regexp.new('\*')
  RE_DV = Regexp.new('\/')

  def rpn
    stack = []
    self.sub(RE_0, "").split(RE_1).each do |t|
      if t =~ RE_D
        stack.push(t.to_f)
        next
      end
      r, l = stack.pop, stack.pop
      case t
      when RE_PL; stack.push(l.to_f + r.to_f)
      when RE_MN; stack.push(l.to_f - r.to_f)
      when RE_PR; stack.push(l.to_f * r.to_f)
      when RE_DV; stack.push(l.to_f / r.to_f)
      end
    end
    return stack.pop
  end
end

if __FILE__ == $0
  f = $stdin.gets.chomp
  exit if f == ""
  puts f.rpn
end