Can't use custom selector as a selector in the ON event

Question

    $.expr[':'].external = function(obj){
       if (! obj.href.match (new RegExp ("^http"))) {
      return (false);
}

return (true);
}

I have a custom jquery selector above. It checks for an external link. I call it using:

 $('a:external')
              .on ('click', function (e) {
        do_stuff ();
    });

Above works great. But below does not. Basically I need to do the below because I load in some content via ajax and I need to check for external links still. I figured the below would work but no luck. If I were to remove the :external from below it works on all links as expected. But adding the selector of :external doesn't produce an event when I click on the link. Any ideas?

$('body')
          .on ('click', 'a:external', function (e) {
        do_stuff ();
});