Number to currency calculator, faster than ActiveSupport (due to no translation?)

example.rb

  def quick_number_to_currency(num)
    left, right = sprintf("%.2f", num).split('.')
    1 while left.sub!(/(\d)(\d\d\d)\b/, '\1' + ',' + '\2')
    "$#{left}.#{right}"
  end

    it "turns 1000 into $1,000.00" do
      @rt.quick_number_to_currency(1000).should eql("$1,000.00")
    end

    it "turns 10000 into $10,000.00" do
      @rt.quick_number_to_currency(10000).should eql("$10,000.00")
    end

    it "turns 100000 into $100,000.00" do
      @rt.quick_number_to_currency(100000).should eql("$100,000.00")
    end

    it "turns 1000000 into $1,000,000.00" do
      @rt.quick_number_to_currency(1000000).should eql("$1,000,000.00")
    end

    it "turns 10000000 into $10,000,000.00" do
      @rt.quick_number_to_currency(10000000).should eql("$10,000,000.00")
    end

    it "turns 100000000 into $100,000,000.00" do
      @rt.quick_number_to_currency(100000000).should eql("$100,000,000.00")
    end

    it "turns 1000000000 into $1,000,000,000.00" do
      @rt.quick_number_to_currency(1000000000).should eql("$1,000,000,000.00")
    end