$re = '/<div class\="custom">(.*?)<\/div>/';
$str = '<div><div class="custom"><p>TEST</p><div>Another inner div</div><div class="another-class">Some text</div></div></div>
<p>Another text</p>
<div><div class="hello"><div class="custom another" data-custom="test-data"><div>Another inner div</div><p>TEST</p><div class="another-class">Some text</div></div></div></div>';
preg_match_all($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