To answer your question:
All VERSION Replies from v4.7 to v5.91 looked like this:
[*]mIRC16 v4.7 K.Mardam-Bey
[*]mIRC32 v5.91 K.Mardam-Bey
And Replies from v6.0 and up look like this:
[*]mIRC v6.12 Khaled Mardam-Bey
There have been no versions released that state "mIRC64" or "(private beta)" in the reply.
Versions before 4.7 had somewhat inconsistant version replies, but should be banned anyway because they're rittled with bugs.
To put this into a faithful regular expression:
[*]/mIRC(?:16|32)? v\d\.\d+ K(?:\.|haled\s)Mardam-Bey/
or more critically
[*]/mIRC(?:16|32) v5\.\d+ K\.Mardam-Bey|mIRC v6\.\d+ Khaled Mardam-Bey/
Also, what EVH suggested is a keen idea.