Where is the petcock on a 98 Buick regal?

The 1998 Buick Regal doesn't have a petcock in the traditional sense like you'd find on a lawnmower or some older vehicles. Fuel delivery is controlled by the fuel pump and the fuel injectors. There's no manually operated valve to drain fuel from the tank. If you need to drain fuel, you'll need to access it through the fuel lines (requiring specialized tools and caution) or by siphoning it. Do not attempt this unless you have experience with automotive fuel systems, as gasoline is extremely flammable.