For a spring stretched past its elastic regime, Hooke's law no longer hold.

For a spring stretched past its elastic regime, Hooke's law no longer hold. As a toy model, we can consider a material whose force changes with displacement as F = -A (1-e-Bx), where A and B are constants that characterize the material. Find the potential energy for this spring, as a function of the displacement, assuming that the unstretched spring has zero potential energy.