To know for sure, you need to:
1. Check your owner's manual: This is the most reliable source. It will specify the exact type of fluid recommended for your specific Regal's transmission.
2. Look at the transmission itself: The transmission type (e.g., 4T65E, etc. for automatics) might be stamped on a tag on the transmission case. You can then search online for the correct fluid for that specific transmission.
3. Check your car's VIN: A mechanic or parts store can use the VIN to determine the exact specifications of your car, including the transmission type.
Without knowing your transmission type, any fluid recommendation is a guess and could damage your transmission. Use the methods above to find the correct information.