$re = '/src="(http:\/\/|https:\/\/)(\S*\.(gif|png|jpg|jpeg))"/i';
$str = '<p><img src="http://cdn03.XXXXXX.de/upload2/Q/m/QmOJ0Li2UbIl143705410063P4391.png"></p>
<p><img src="http://cdn02.XXXXXX.de/upload2/i/Q/iQtReUOqw1Jf143705410914P4391.gif"></p>
<p><img src="http://cdn03.XXXXXX.de/upload2/5/D/5Dse28yAh160143705411823P4391.jpg"></p>';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php