Código PHP:
<?php
$html = '<a href="http://imageshack.us/photo/my-images/29/lovelysky.jpg/" target="_blank">
<img src="http://imageshack.us/a/img29/599/lovelysky.th.jpg" border="0"/></a>';
$exprRegular = '#\/[a-z]\/[a-z]{3}[0-9]{2}\/[0-9]{3}\/[a-z]+\.[a-z]{2}\.[a-z]{3}#';
$result = preg_match($exprRegular,$html,$matches);
echo $matches[0];
?>
Espero te sirva.!
