Exactly my experience on this. In a past life I've had to parse valid HTML that was generated by a forum system; the user submitted something akin to bbcode [b]

Exactly my experience on this. In a past life I've had to parse valid HTML that was generated by a forum system; the user submitted something akin to bbcode [b]this sort of thing[/b] that was pre-parsed and converted to valid HTML, and then I had to parse that fragment again after the fact. Given the constraints it's entirely possible to parse (a subset of) irregular grammar with regular expressions. Asking a question along those lines on SO would have have only elicited responses that I/someone was doing it wrong. I won't argue that it was or wasn't the wrong to do, but you don't always get to pick your client.