filter, find

http://manalang.com/jquery/docs/index.html#filter-expr
http://www.learningjquery.com/2008/08/quick-tip-dynamically-add-an-icon-for-external-links

      $(document).ready(function() {
        $('#extlinks a').filter(function() {
          return this.hostname && this.hostname !== location.hostname;
        }).after(' <img src="/images/external.png" alt="external link">');
      });

filter 可以塞function來parse, return true 才會跑後面的after