Anyone who attempts to generate random numbers
    by deterministic means is, of course, living in
    a state of sin.

        -- John von Neumann

Modified to:

    Anyone who attempts to validate email addresses
    by a regular expression is, of course, living in
    a state of sin.

        -- Any sensible person

How many '@' characters are possible in a valid address?

I don't care enough about offline email validation to figure it out carefully, but I have the idea it is one.