You can't really hide it with regex and what you're suggesting looks a lot like a script that will force a user to always be in some channel of your choosing, which is rather annoying, imo. If you really wanted to, you could make the regex really complicated so that it does all kinds of checks and so on to come up with a simply /join #channel command, but that would be slow and cumbersome and wouldn't really hide it... just make it confusing.