calc.rb

class Calculator
  def exec(expression)
    while op = expression.match(/\([^()]+\)/)
      offsets = op.offset(0)
      out = exec op[0].gsub(/[()]/,'')
      expression[offsets.first...offsets.last] = out.to_s
    end

    while op = expression.match(/\d+ ?[\*\/] ?\d+/)
      run_expression op, expression
    end

    while op = expression.match(/\d+ ?[\+\-] ?\d+/)
      run_expression op, expression
    end

    expression.to_i
  end

  private

  def run_expression(expression, full_expression)
    offsets = expression.offset(0)
    first, operator, second = expression[0].match(/(\d+) ?([+\-*\/]) ?(\d+)/).to_a[1..-1]
    # p "#{first} #{operator} #{second}"
    first = first.to_i
    second = second.to_i
    output = case operator
             when "+"
               first + second
             when "-"
               first - second
             when "*"
               first * second
             when "/"
               first / second
             end
    full_expression[offsets.first...offsets.last] = output.to_s
  end
end

calc_spec.rb

RSpec.configure do |config|
  config.color_enabled = true
end

require File.join(File.dirname(__FILE__), 'calc')

describe Calculator do
  before(:each) do
    @calc = Calculator.new
  end

  it "should return 2" do
    @calc.exec("1 + 1").should == 2
  end

  it "should return 4" do
    @calc.exec("1 + 1 + 2").should == 4
  end

  it "should return 3" do
    @calc.exec("1 + 1 * 2").should == 3
  end

  it "should return 2" do
    @calc.exec("1 + 1 * 2 - 1").should == 2
  end

  it "should return 8" do
    @calc.exec("(2 * 5) - (1 * 2)").should == 8
  end

  it "should return 4" do
    @calc.exec("(((2 * 5) - (1 * 2)) / (11 - 9))").should == 4
  end
end