About all i can tell you is that afaik, mIRC uses the PCRE (Perl Compatible Regular Expressions) library,
web page You may find the tutorial at this
web page of interest.
You could also use the search feature here, all forums, all posts with regex as search word for any threads on the subject (prolly most are in the scripting area)
Hope this helps.... sorry, but when it comes to regex or perl i am virtually clueless