Well since the // are required at the start and end of the matchtext, and these characters are not valid for the next parameter (where the text matches, i.e. in a channel or whatever) then there should be no reason why these symbols are not allowed for regex matching.

Oh, and at the start of this thread, mIRC bug #2 about /hdec working as /hinc which was posted by me has been graciously deleted without notice. Regards to whoever did that.