A doppelgänger does not have the Regeneration special quality. It does have Change Shape (alter self).
Regeneration clearly lets a creature regrow a missing body part, like a hand.
What happens when a doppelgänger in one shape (say, an elf) has a right hand cut off as punishment for stealing, then slinks out of sight and changes shape into a human?
Is the doppelgänger in human form still missing the right hand?
If a hand has to be missing, can the doppelgänger take the form of a human missing its left hand? Hide or disguise the wound to a degree?