Foreign key check only if not null?

Hi,

I bet this is a simple solution but I have been racking my brains.

I have a column in my table:

user_id varchar(100) ;

This can be NULL, or it can have a value. If it has a value during
INSERT or UPDATE, I want to check that the user exists against my
"Users" table. Otherwise, NULL is ok. (Because the functionality in
question is open to both unregistered and registered users).

Any idea on how I can implement a FOREIGN KEY constraint? Or do I need
a pre-insert and pre-update RULE for this?

Thanks!