$re = '/(?<FirstPart>(?:<div>.*?){4})|\G(?<!^).*\K(?<SecondPart>(?:<\/div>.*?){4})/is';
$str = '<!DOCTYPE html>
<html>
<head>
</head>
<body>
<div>
<div>
<div>
<div>
<div>
<div>
<div>
<div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</body>
</html>';
preg_match_all($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