Ruby class to add your Amazon Associates referral id to an Amazon URL, replacing any that were already there. See http://github.com/henrik/delishlist.com/blob/master/lib/amazon_referralizer.rb for the latest version.

amazon_referralizer.rb

# Amazon::Referralizer
# by Henrik Nyh <http://henrik.nyh.se> 2010-07-05 under the MIT license.

# Add your Amazon Associates referral id to an Amazon URL, replacing any that were already there.
#
# E.g.:
#
#   Amazon::Referralizer.referralize("http://amazon.co.uk/o/ASIN/B00168PO6U/evil-20", "good-20")
#   # => "http://amazon.co.uk/o/ASIN/B00168PO6U/good-20"
#

class Amazon
  class Referralizer

    ASIN_RE = /[A-Z0-9]{10}/
    REFERRAL_ID_RE = /\w+-\d\d/
    AMAZON_URL_RE = %r{^https?://(.+\.)?(amazon\.|amzn\.com\b)}i
    # http://daringfireball.net/2010/07/improved_regex_for_matching_urls
    GENERAL_URL_RE = %r{(?i)\b((?:https?://|www\d{0,3}[.]|[a-z0-9.\-]+[.][a-z]{2,4}/)(?:[^\s()<>]+|\(([^\s()<>]+|(\([^\s()<>]+\)))*\))+(?:\(([^\s()<>]+|(\([^\s()<>]+\)))*\)|[^\s`!()\[\]{};:'".,<>?«»“”‘’]))}
  
    def self.referralize(url, tag)
      url = url.to_s

      if url.match(AMAZON_URL_RE)
        url.sub(%r{\b(#{ASIN_RE})\b(/#{REFERRAL_ID_RE}\b)?}, "\\1/#{tag}").
            gsub(/&tag=#{REFERRAL_ID_RE}\b/, '').
            gsub(/\?tag=#{REFERRAL_ID_RE}\b/, '?').sub(/\?$/, '')
      else
        url
      end
    end

    def self.referralize_all(text, tag)
      text.gsub(GENERAL_URL_RE) { self.referralize($&, tag) }
    end

  end
end


if __FILE__ == $0
  require "test/unit"
  
  class AmazonReferralizerTest < Test::Unit::TestCase

    def assert_conversion(expected_url, input_url)
      assert_equal(expected_url, Amazon::Referralizer.referralize(input_url, "test-20"))
    end
    
    def test_url_formats
      assert_conversion "http://amazon.com/gp/product/B00168PO6U/test-20",
                        "http://amazon.com/gp/product/B00168PO6U"
      assert_conversion "http://amazon.com/exec/obidos/tg/detail/-/B00168PO6U/test-20",
                        "http://amazon.com/exec/obidos/tg/detail/-/B00168PO6U"
      assert_conversion "http://amazon.com/o/ASIN/B00168PO6U/test-20",
                        "http://amazon.com/o/ASIN/B00168PO6U"
      assert_conversion "http://amazon.com/dp/B00168PO6U/test-20",
                        "http://amazon.com/dp/B00168PO6U"
    end
    
    def test_international
      assert_conversion "http://amazon.co.uk/o/ASIN/B00168PO6U/test-20",
                        "http://amazon.co.uk/o/ASIN/B00168PO6U"
      assert_conversion "http://amazon.de/o/ASIN/B00168PO6U/test-20",
                        "http://amazon.de/o/ASIN/B00168PO6U"
    end
    
    def test_short
      assert_conversion "http://amzn.com/B00168PO6U/test-20",
                        "http://amzn.com/B00168PO6U"
    end
    
    def test_keep_ref
      assert_conversion "http://www.amazon.com/dp/B00168PO6U/test-20/ref=cm_sw_su_dp",
                        "http://www.amazon.com/dp/B00168PO6U/ref=cm_sw_su_dp"
    end
    
    def test_replace_other_tag_in_path
      assert_conversion "http://www.amazon.com/dp/B00168PO6U/test-20/ref=cm_sw_su_dp",
                        "http://www.amazon.com/dp/B00168PO6U/evil-20/ref=cm_sw_su_dp"
    end
    
    def test_remove_other_tag_in_query_string
      assert_conversion "http://www.amazon.com/dp/B00168PO6U/test-20/ref=cm_sw_su_dp",
                        "http://www.amazon.com/dp/B00168PO6U/ref=cm_sw_su_dp?tag=evil-20"
      assert_conversion "http://www.amazon.com/dp/B00168PO6U/test-20/ref=cm_sw_su_dp?one=1&two=2",
                        "http://www.amazon.com/dp/B00168PO6U/ref=cm_sw_su_dp?one=1&tag=evil-20&two=2"
    end

    def test_referralize_all
      input    = "Hello http://google.com and http://amazon.com and http://amazon.com/gp/product/B00168PO6U/evil-20 and http://amzn.com/B00168PO6U!"
      expected = "Hello http://google.com and http://amazon.com and http://amazon.com/gp/product/B00168PO6U/test-20 and http://amzn.com/B00168PO6U/test-20!"

      assert_equal(expected, Amazon::Referralizer.referralize_all(input, "test-20"))
    end
  
  end

end

Tack så hämsk mycket, just vad jag behøvde!