I know it is probably gauche, but here's a tl;dr: The "regexp" in your programming language isn't "regular expression" from language theory. Adding extra bells and whistles (like back references) gives "regexps" power at least equivalent to context-free languages.

This is an excellent article, and I hope the tl;dr makes you more likely to go and read it.

Best answer ever why you shouldn't parse HTML with regex:

http://stackoverflow.com/questions/1732348/regex-match-open-...