Reset dvd drive region When you setup your dvd drive for the first time it will ask you what reigen you want to set. 

Each time you put a dvd from a different reigen into the drive it will ask you to change again. 

But you are only allowed to do this 4 times, once you've exhausted that is there any way to change it back ? Or are you now stuck - ie. not able to play dvd's from any other region ?
Unless you are willing to research how to flash the firmware of the particular DVD drive in your Mac to change the region, or to make it region-free, I suggest you try the very cool (and free) app VLC Media Player from VideoLAN. If VLC does what you need it to, please consider donating to the 'cause'.

Your milage may vary. 

Disclaimer: I have no financial stake in VideoLAN.org - I'm just a very satisfied customer.