Using Chtmlpurifier Safeiframe

Yii version:1.1.13 (just in case it matters)

I am trying to allow YouTube iFrame code in blog posts.

//Allow HTML target attribute and YouTube iFrame
$content = $data->content;
$p = new CHtmlPurifier();
$p->options = array(
	'Attr.AllowedFrameTargets'=> array('_blank'), 
	'HTML.Allowed'=> 'p,a[href|target],strong,em,br',
	'HTML.SafeIframe'=> true,
	'URI.SafeIframeRegexp'=>'%^http://(www.youtube.com/embed/)%',
);
$content = $p->purify($content);

The YouTube iFrame code

<iframe width="560" height="315" src="http://www.youtube.com/embed/aXPP2SQuGSM?rel=0" frameborder="0" allowfullscreen></iframe>

does not get rendered into the post.

What am I doing wrong?

Thanks.