We have a 99 regal, and the anti-theft is keeping the car from starting, do you know what we could try to get it working?

what sensor ????? No sensor , Is the anti-theft light flashing when trying to start the vehicle ? Anti-theft system disables the fuel injectors - no fuel into engine . Your best bet , take it to a qualified repair shop.

Anti-theft system can keep vehicle from starting , Alarm is for vehicle content theft , open door without key alarm sounds (horns) ! PATS - passive anti theft system PATS enables and disables the engine from starting ,it will not cause the engine to die ! You have some other kind of problem ! If the vehicle engine doesn't turn over an red light on top of dash flashes fast ,then you have anti-theft system problem !
PATS differs from the perimeter anti-theft system in that PATS enables and disables the engine from starting. If equipped, the perimeter anti-theft system protects the perimeter of the vehicle (doors, hood and trunk) and sounds an alarm.

flat battery , causing the anti theft system to cut in would be my first guess, when changing the battery make sure at least one door is open as when reconnecting new battery the anti theft will most likely lock the vehicle

Is your SECURITY light flashing? If so, you have a V.A.T.S. problem. VATS stands for Vehicle Anti Theft System. Try getting a new ignition key made because the resistance value has probably changed in the one you have. If that doesn't do it you'll need to get a VATS bypass.

could possibly be anti-theft problem with key or vats system.
is your security light flashing. if you have questions , call or text me
at 270-556-7357 mobile phone

Absolutely! If the Passive Anti-Theft System (PATS) "thinks" you are trying to steal the car, it is SUPPOSED to keep it from starting. The problem is that the PATS thinks you are a car theif when you are not.
Below is a table straight out of the service manual for your vehicle. Please note that the FIRST action that must be taken for any PATS system malfunction is to scan the computer system for fault codes.

you could pull the fuse disabling th anti theft totally.. not sure that would work though

Clean battery cables and replace cable bolts.

I think the anti theft anewer is correct. I have an extra key for my '99 Lincoln continental that apparently doesnt have whatever "chip" it needs in it. If you try to use that key it cranks but won't start. I don't know if the key "chip" can fail or not. Maybe a quick conversation with a lock smith will shed some light on the problem.

I believe it is the 3 and 5 buttons on the radio it will bring up the #'s keep pushing them until you get your code all typed in.