If I want to use batteries (a cluster of four AAs would be ideal) to generate a stable 5VDC power supply...?

What voltage regulator would I need to make the best use of my battery pack? Note that AA batteries as they are used will drop in voltage, so when the batteries are fresh, the regulator would be receiving 1.5VDC...and towards the end of the battery life it would drop to less than 1VDC. Yes, I know I could hook them up in parallel to get 6VDC to start and 4VDC towards the end of their lifespan, but I want to step up to 5VDC...and using this in parallel makes it easy to recharge the AAs (if I'm using rechargable batteries) via a computer's USB port.