I think even for single opening tags like asked there are impossible edge cases.

For example, this is perfectly valid XHTML:

    <a href="/" title="<a /> />"></a>

▲ comex 2 hours ago | parent [-]

If you already know where the start of the opening tag is, then I think a regex is capable of finding the end of that same opening tag, even in cases like yours. In that sense, it’s possible to use a regex to parse a single tag. What’s not possible is finding opening tags within a larger fragment of HTML.

	▲	kstrauser 19 minutes ago \| parent [-]
		For any given regex, an opponent can craft a string which is valid HTML but that the regex cannot parse. There are a million edge cases like: `<!—- Don't count <hr> this! -—> but do count <hr> this -->` and `<!-- <!-- Ignore <ht> this --> but do count <hr> this —->` Now your regex has to include balanced comment markers. Solve that You need a context-free grammar to correctly parse HTML with its quoting rules, and escaping, and embedded scripts and CDATA, etc. etc. etc. I don't think any common regex libraries are as powerful as CFGs. Basically, you can get pretty far with regexes, but it's provably (like in a rigorous compsci kinda way) impossible to correctly parse all valid HTML with only regular expressions.