[danik.js]

var url = "http://www.youtube.com/watch?v=_BbniyMqAQQ";

var matches = url.match( /^(?:https?:\/\/)?(?:www\.)?youtube\.com\/watch\?(?=.*v=((\w|-){11}))(?:\S+)?$/);

var result = matches ? matches[1] : null;

alert (result);