Reverse Polish Notation Calculator

RPN.rb

# The following method will take strings like '5 4 +', '-1', '5 -3 4 + 6 9 - * +', and '2 3 4 7 + * -', and return the result of running the calculation.
# Note: when performing division it will always round down.

def evaluate(string)
  return string.to_i unless string.include?(' ')
  string.match /(-?\d+) (-?\d+) ([-+*\/])(?!\d)/
  result = ($1.to_i).send($3,$2.to_i)
  string.gsub!($1 + ' ' + $2 + ' ' + $3, result.to_s)
  evaluate(string)
end