i suck at remembering regex, so i always use dom->xpath queries instead. you could find the tag with a query something like this:
<?php
$dom = new DOMDocument();
$dom->loadHTML(HTMLHERE);
$xpath = new DOMXpath($dom);
$selectH1 = $xpath->query('//h1[@class="classnamehere"]');
?>
just came up with that right now, so dunno if that's the exact syntax, but i prefer traversing html w/ dom rather than regex
this is a good xpath query guide:
http://www.earthinfo.org/xpaths-with-php-by-example/