"thumb":"http:\/\/cs510320.vk.me\/u30156798\/video\/l_3c803d1c.jpg"
http://cs510320.vk.me/u30156798/video/l_3c803d1c.jpg
$str = '"thumb":"http:\/\/cs510320.vk.me\/u30156798\/video\/l_3c803d1c.jpg"';
preg_match('/"(?P<key>)\w+":"(?P<val>.+)"/', $str, $mat);
echo str_replace("\/", "/", $mat['val']);