parse string of arithmetic operation and perform the operations

arithmetic_opp.rb

input = "20+4+2-1"

class String
    def is_i?
       !!(self =~ /\A[0-9]+\z/)
    end
end

def operation(op)
  case op
  when "+" then -> (a, b){a + b}
  when "-" then -> (a, b){a - b}
  else
    p a,b,op
    raise "no operation defined"
  end
end

tokens = input.split(/(\d*)([-+])/).reject { |e| e.empty? } # => ["2", "+", "4", "+", "2"]
operaions_stack = tokens.map { |token| token.is_i? ? token.to_i : operation(token)} # => array of numbers and lambdas containing the operation to be performed

# inject an empty stack, and when stack size is 3 compute the result and put it back in stack
last_opp = operaions_stack.inject([]) do |mem, var|
  mem = [mem.first.(mem[1], mem[2])] if mem.size == 3
  if var.is_a? Proc
    mem.unshift(var)
  else
    mem << var
  end
end
p last_opp.first.(last_opp[1], last_opp[2])