There is already a built-in function to do what you want. No need for regexp and other dirty solutions.
$thumbnail_id = get_post_thumbnail_id($post->ID);
$thumbnail_object = get_post($thumbail_id);
After that you can just call things like $thumbnail_object->guid (this is the src to the image) and $thumbnail_object->post_title. Try running an print_r($thumbnail_object) and you’ll see what other goodies are stored there ??