flip_flop.rb

require "minitest/unit"
require 'minitest/autorun'

def calculate(str)
	acc = state = 0
	op = "+"
	str.scan(/(?:.+?\b)/) do |atom|
		if (state = 1 - state) == 1 # number state
			acc = acc.send(op, Integer(atom))
		else # op state
			op = atom
		end
	end
	
	return acc
rescue ArgumentError
	return nil
end

class TestCalculate < MiniTest::Unit::TestCase
	def test_calculate
		assert_equal 9, calculate("4+5")
		assert 20, calculate("10+8+2")
		assert 5, calculate("10-5")
		assert 9, calculate("10+2-4")
		assert_nil calculate("12**12**1fsdaf2")
	end
end

test_output.txt

Run options: --seed 35201

# Running tests:

. 

Finished tests in 0.001561s, 640.6150 tests/s, 3203.0750 assertions/s.

1 tests, 5 assertions, 0 failures, 0 errors, 0 skips

def calculate(str)
  eval(str) if str[/\A[\s\d+\-\/\*]+\z/]
end

Eval wasn't allowed!