On the topic of Regeneration of limbs - generally, you need the limb itself and re-attach it with a regeneration/healing spell. That's usually how it works in D&D. So, say you got your head chopped off in combat, you'd need your head, and your body, and stick 'em together and heal. Same goes for arms, legs, etc, unless by miraculous resurrection, which, by my understanding, can reform a body from ashes.