reverse_polish_notation.rb

#!/usr/bin/env ruby

while $_ = gets
  a ||= []
  if /\d/ ~= $_
    a << $_.to_i
  end
  if /([-+*\/])/ ~= $_
    a << eval([a.pop, a.pop].reverse.join($1)
    puts a[-1]
  end
end

# klm is cool
# ruby -ne'a||=[];~/\d/&&a<<$_.to_i;~/([-+*\/])/&&a<<eval([a.pop,a.pop].reverse.join $1)and puts a[-1]'