Reverse Polish Notation Calculator. Can do recursively?

gistfile1.txt

class RPNCalculator
  
  def evaluate(input)
    puts "BEGIN NEW CALC"
    stack = [] 
    puts "Initial input is #{input}"
    until input == ""
      item = input.match(/\A\S*/)
      puts "Current item is #{item}"
      item_string = item.to_s
      test = item_string.match(/\d/)
      if test
        num_item = item_string.to_f
        stack.push(num_item)
        puts "Added #{item} to the stack"
      elsif item_string == '+'
        b = stack.pop
        a = stack.pop
        sum = a + b
        stack.push(sum)
      elsif item_string == '-'
        b = stack.pop
        a = stack.pop
        diff = a - b
        stack.push(diff)
      elsif item_string == '*'
        b = stack.pop
        a = stack.pop
        prod = a * b
        stack.push(prod)
      else
        puts "WARNING!!"
       return "Error: unknown input #{item} Remaining input is #{input}"
      end
      puts "Made it through the loop"
      size = item_string.length
      chops = size + 1
      chops.times do
        input = input.reverse.chop.reverse
      end
      puts "Remaining input is #{input}"
    end
    puts "Exited the loop"
    puts stack.inspect
    puts "The result is #{stack[0]}"
    return stack[0]   
  end
  
end