$re = '/(?<=<!--dle_image_begin:)((?:(?!\|-->).)*)(?=|-->)/';
$str = '<!--dle_image_begin:http://some_url.com/uploads/posts/2012-12/1.png|--><img src="some_url.com/uploads/posts/2012-12/1.png" alt="***" title="***" /><!--dle_image_end--> bla bla bla bla <!--dle_image_begin:http://some_url.com/uploads/posts/2012-12/2.png|--><img src="some_url.com/uploads/posts/2012-12/2.png" alt="***" title="***" /><!--dle_image_end--> bla bla bla <!--dle_image_begin:http://some_url.com/uploads/posts/2012-12/3.png|--><img src="some_url.com/uploads/posts/2012-12/3.png" alt="***" title="***" /><!--dle_image_end-->';
preg_match_all($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