No, a 1997 Buick Century transmission will almost certainly
not fit in a 1997 (or similar year) Buick Regal. While both are Buick's and from the same era, they used different transmissions depending on the engine and other options. Even if the external dimensions appeared similar, the bolt patterns, internal components, and computer interfaces would likely be incompatible.
Trying to force a fit would be a significant undertaking and likely impossible without extensive, and probably uneconomical, modification. You'd need to find the correct transmission for the specific Regal's engine and year.
