I wish OP had provided some actual A/B registration fall-off data. For me client-side validation is more for catching user error than to do any actual "validation". That's always been the job of confirmation email.

I think there is going to be a very low percentage of users who will register again if they don't receive a confirmation email, unless you're giving away free iPhones. Granted regex can not catch most of the user-generated errors, but it can catch a few which could still increase you registered users.

To that point a better UI/UX (font size, spacing, etc.) might do a better job in lowering typos in email.