$re = '~(<h3>.*?</h3>)((?:my-class(*SKIP)(*F)|.)*?)(<p>.*?</p>)~is';
$str = '<h3>Header</h3>
<img />
<p>some text</p>
нужно чтобы он пропустил это
<h3>Header</h3>
<img />
<div class="my-class">content</div>
<p>some text</p>
<h3>Header</h3>
<img />
<p>some text</p>
';
preg_match_all($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