A standard troll has Regeneration 5 (acid or fire). Suppose that the troll somehow gains fire resistance 10, such as through the Resist Energy spell. If a level 3 wizard casts Burning Hands at this troll and rolls 8 on his 3d4 damage dice, does it shut off the troll's regeneration for the round?
The text of the Regeneration ability reads in part (some emphasis added):
A creature with this ability is difficult to kill. Creatures with regeneration heal damage at a fixed rate, as with fast healing, but they cannot die as long as their regeneration is still functioning (although creatures with regeneration still fall unconscious when their hit points are below 0). Certain attack forms, typically fire and acid, cause a creature’s regeneration to stop functioning on the round following the attack. During this round, the creature does not heal any damage and can die normally. The creature’s descriptive text describes the types of damage that cause the regeneration to cease functioning.
Normally, I would say that the fire has to reduce the troll's HP in order to shut off its regeneration. However, the term "attack forms" has me a bit uncertain.