$re = '/<div\b(?:(?R)|(?:(?!<\/?div).))*<\/div>/m';
$str = '<div class="main">asdasd<div>jk<div>jkjk</div>jk</div></div>';
preg_match_all($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