[IMGate] RegExp Question

[Russ Uhte (Lists)]

I've just started using IMGate, and so far am very pleased.  I've read the 
manuals, yes read, yes all of them :)  some of the more relevant ones 
twice.  What I still don't understand is RegExp.  I'm trying to figure out 
exactly what I'm denying in the header_checks.regexp file (default basic 
install).  If someone knows where I can find _good_ information on RegExp, 
I'd be more than happy to RTFM!!
Thanks,
Russ