How can I power a 3.3v circuit with a 9V power supply and two AA batteries?

I have a circuit that runs on 3.3v. I want it to be able to operate on two AA batteries down to about 2.2v (or until batteries are dead), and also with a 9V power supply. When the power supply is plugged in, the batteries should be disconnected. The power jack being used has a switching ability, and would work fine if both power sources were at the same voltage. But when battery power is active, t