SRD:Regeneration

Regeneration (Ex)
A creature with this extraordinary ability is difficult to kill. Damage dealt to the creature is treated as nonlethal damage. The creature automatically heals nonlethal damage at a fixed rate per round, as given in the entry. Certain attack forms, typically fire and acid, deal lethal damage to the creature, which doesn’t go away. The creature’s descriptive text describes the details. A regenerating creature that has been rendered unconscious through nonlethal damage can be killed with a coup de grace. The attack cannot be of a type that automatically converts to nonlethal damage. An attack that can cause instant death only threatens the creature with death if it is delivered by weapons that deal it lethal damage.

Attack forms that don’t deal hit point damage ignore regeneration. Regeneration also does not restore hit points lost from starvation, thirst, or suffocation. Regenerating creatures can regrow lost portions of their bodies and can reattach severed limbs or body parts; details are in the creature’s descriptive text. Severed parts that are not reattached wither and die normally.

A creature must have a Constitution score to have the regeneration ability.