[Serusers] Registration restrictions

[Tamas J]

Hello,

I would like to restrict registrations to use username, auth. username
and password fields all together.
My problem:
If the user sets in his device the auth. username and password, it's
enough for registration. However he can set whatever username, which I
don't want to allow, while I would like to use numbers and don't want
that anybody misuse this.

Any idea how to do that?

Kind regards,
        Tamas