Does anyone have any idea (using preg_match or preg_match_all) on how to look at a web page and see which html tags are being used?

So, looking at this page, the function would return a big array containing h1, h1, p, i, table etc. as its values.

Thanks to anyone that can help.

Better than using regexp, you should use the php's DOM extension for that:
http://www.php.net/manual/en/ref.dom.php
http://www.bluehatseo.com/guest-post...inks-with-php/

Thanks.

Do the DOM extensions require the HTML to be valid?

No:

The function parses the HTML contained in the string source. Unlike loading XML, HTML does not have to be well-formed to load.

http://www.php.net/manual/en/functio...t-loadhtml.php