gistfile1.txt

  VALID_URLS = [
     /^(?:http:\/\/)?(?:www\.)?youtube\.com\/watch\?v=([a-zA-Z0-9_-]*)/,
     /^(?:http:\/\/)?(?:www\.)?vimeo\.com\/(\d+)/,
     /^(?:http:\/\/)?(?:www\.)?soundcloud\.com\/(\w+)/,
     nil
   ]                  
  
  validates :link, :uniqueness => true,
                   :format => { :with => Regexp.new(VALID_URLS.join('|')) }