I've looked before, and if they're out there they certainly don't practice SEO. (Get your right-click ready, pop-ups galore!)
[javaworld.com...]
[anso.virtualave.net...]
<added>
Ultraedit has a fair overview of RegExp in it's help files, I turn to that quite often.
A very good resource is O'Reilly's Mastering Regular Expression.
You'll have to pay for them, but like all O'Reilly books (is there a useless O'Reilly book??) they're worth their weight in gold...
regex by example -
[phpbuilder.com ]
regex quick guide -
[contactor.se ]
php regex -
[developers-resources.com ]
one more -
[ciser.cornell.edu ]