Restricting email by regexp at registration : CB Auto Action how ?

I'd like to restrict the email given at registration by a regexp (covering not only the domain).

How can I achieve this with Auto Action ?
- Which Type / Event ?

(choosing Register, I was asked username, email, etc. which is not available at registration : I don not want to act after registration, but before and not accept email that do not comply)

Custom regex validation combined with forbidden words should work just fine. You can configure both while editing your email field. Aside from that you need to be more specific for me to advise any further.