I'm not an expert with regex, but a alternative way would be to use explode on the " marks and get array[1] like this:
$rssFeed = '<div>
<p>
Some text
</p>
<iframe src="http://www.source.com"></iframe>
</div>';
$rssArray = explode('"', $rssFeed);
echo $rssArray[1];
This requires your RSS feed to be very consistent though, if the "Some text" part were to contain " marks, this would mess up and you'd get a wrong string.
You could look through the array for everything starting with http or www to work around errors, but again, it requires a very consistent RSS feed, so you have to judge for you self if this would do the job good enough.