DIEHARD
When reduced to negative hit points, you may choose to act as if you were disabled, rather than dying.
When you take nonlethal damage, keep a running total of how much you’ve accumulated. Do not deduct the nonlethal damage number from your current hit points. It is not “real” damage. Instead, when your nonlethal damage equals your current hit points, you’re staggered, and when it exceeds your current hit points, you fall unconscious. [emphasis in original]
Regeneration (Ex): A creature with this ability is difficult to kill. Damage dealt to the creature is treated as nonlethal damage.