I couldn't help thinking while reading this that my main concern with registration systems isn't the complicated Regex as much, but rather the really annoying registry bots that sign up phantom accounts. I'm not a huge Captcha fan, but without something (Recaptcha, ghosted fields, etc) you'll get SPAMMED with tons of fake accounts - and they have valid emails, AND the bots click on the links in the confirmation email automatically. It's really sad.