If Mossbridge Troll would be destroyed, regenerate it.
Regeneration is defined as:
419.6b Regeneration is a destruction-replacement effect. The word "instead" doesn't appear on the card but is implicit in the definition of regeneration. "Regenerate [permanent]" means "The next time [permanent] would be destroyed this turn, instead remove all damage from it, tap it, and (if it's in combat) remove it from combat." Abilities that trigger from damage being dealt still trigger even if the permanent regenerates.
Mossbridge Troll's ability is a replacement ability in and of itself. Hence, it replaces "Destroy Mossbridge Troll" with "The next time [permanent] would be destroyed this turn, instead remove all damage from it, tap it, and (if it's in combat) remove it from combat." Instead of REGENERATING (i.e, tapping and exiting combat), destruction events are replaced by a REGENERATION SHIELD.
TL, DR version: Mossbridge Troll's first ability, if not errataed, does not cause it to tap, or get removed from combat. Instead, if you "destroy" it, it gets a regeneration shield. This is BIZARRE.
[Extra spicy bizarre, actually. Lethal damage would cause a destruction effect, which would be replaced by a regeneration shield, but the lethal damage would not be removed. SBEs would check again, find lethal damage, and re-create a regeneration shield, repeat ad infinitum, drawing the game. My errata-spidey sense is tingling.]
It's not a replacement effect. It does not replace the event of the Mosswort Troll being destroyed, it simply triggers before it is destroyed. The Regeneration effect does the replacing, not the trigger on the Troll's ability.
If it were a replacement effect, it would say "If Mosswort Troll would be destroyed, regenerate it instead."
Mossbridge Troll's ability is a replacement ability in and of itself. Hence, it replaces "Destroy Mossbridge Troll" with "The next time [permanent] would be destroyed this turn, instead remove all damage from it, tap it, and (if it's in combat) remove it from combat." Instead of REGENERATING (i.e, tapping and exiting combat), destruction events are replaced by a REGENERATION SHIELD.
501.5a If the effect of a resolving spell or ability regenerates a permanent, it creates a replacement effect that protects the permanent the next time it would be destroyed this turn. In this case, “Regenerate [permanent]” means “The next time [permanent] would be destroyed this turn, instead remove all damage from it, tap it, and (if it’s in combat) remove it from combat.”
501.5b If the effect of a static ability regenerates a permanent, it replaces destruction with an alternate effect each time that permanent would be destroyed. In this case, “Regenerate [permanent]” means “Instead remove all damage from [permanent], tap it, and (if it’s in combat) remove it from combat.”
The second citation should explain a bit more about how this functions. As worded, provided nothing funny happens with the rules, Moosbridge Troll's ability is a static ability that creates the replacement effect (i.e., regeneration). It doesn't trigger or do anything just before the event, it just generates a replacement effect which will be applied before that event ever occurs.
501.5a If the effect of a resolving spell or ability regenerates a permanent, it creates a replacement effect that protects the permanent the next time it would be destroyed this turn. In this case, “Regenerate [permanent]” means “The next time [permanent] would be destroyed this turn, instead remove all damage from it, tap it, and (if it’s in combat) remove it from combat.”
501.5b If the effect of a static ability regenerates a permanent, it replaces destruction with an alternate effect each time that permanent would be destroyed. In this case, “Regenerate [permanent]” means “Instead remove all damage from [permanent], tap it, and (if it’s in combat) remove it from combat.”
The second citation should explain a bit more about how this functions. As worded, provided nothing funny happens with the rules, Moosbridge Troll's ability is a static ability that creates the replacement effect (i.e., regeneration). It doesn't trigger or do anything just before the event, it just generates a replacement effect which will be applied before that event ever occurs.
Ah, I get it. We're changing the definition of "Regenerate ~foo~" depending on the type of ability it appears on. That's... Uh... Well, that WORKS, but it's damn ugly.
If Mossbridge Troll would be destroyed, regenerate it.
Regeneration is defined as:
419.6b Regeneration is a destruction-replacement effect. The word "instead" doesn't appear on the card but is implicit in the definition of regeneration. "Regenerate [permanent]" means "The next time [permanent] would be destroyed this turn, instead remove all damage from it, tap it, and (if it's in combat) remove it from combat." Abilities that trigger from damage being dealt still trigger even if the permanent regenerates.
Mossbridge Troll's ability is a replacement ability in and of itself. Hence, it replaces "Destroy Mossbridge Troll" with "The next time [permanent] would be destroyed this turn, instead remove all damage from it, tap it, and (if it's in combat) remove it from combat." Instead of REGENERATING (i.e, tapping and exiting combat), destruction events are replaced by a REGENERATION SHIELD.
TL, DR version: Mossbridge Troll's first ability, if not errataed, does not cause it to tap, or get removed from combat. Instead, if you "destroy" it, it gets a regeneration shield. This is BIZARRE.
[Extra spicy bizarre, actually. Lethal damage would cause a destruction effect, which would be replaced by a regeneration shield, but the lethal damage would not be removed. SBEs would check again, find lethal damage, and re-create a regeneration shield, repeat ad infinitum, drawing the game. My errata-spidey sense is tingling.]
If it were a replacement effect, it would say "If Mosswort Troll would be destroyed, regenerate it instead."
This is not new or groundbreaking, as Knight of the Holy Nimbus and Clergy of the Holy Nimbus feature similar text. Hence why the following rules were added to the Comprehensive Rules:
501.5. Regenerate
501.5a If the effect of a resolving spell or ability regenerates a permanent, it creates a replacement effect that protects the permanent the next time it would be destroyed this turn. In this case, “Regenerate [permanent]” means “The next time [permanent] would be destroyed this turn, instead remove all damage from it, tap it, and (if it’s in combat) remove it from combat.”
501.5b If the effect of a static ability regenerates a permanent, it replaces destruction with an alternate effect each time that permanent would be destroyed. In this case, “Regenerate [permanent]” means “Instead remove all damage from [permanent], tap it, and (if it’s in combat) remove it from combat.”
The second citation should explain a bit more about how this functions. As worded, provided nothing funny happens with the rules, Moosbridge Troll's ability is a static ability that creates the replacement effect (i.e., regeneration). It doesn't trigger or do anything just before the event, it just generates a replacement effect which will be applied before that event ever occurs.
Ah, I get it. We're changing the definition of "Regenerate ~foo~" depending on the type of ability it appears on. That's... Uh... Well, that WORKS, but it's damn ugly.
Remember, kids: Never fight with Flashback, 'cause Flashback always wins.
They were included with the Future Sight Comprehensive Rules update in May 2007, when "keyworded actions" were added. See Yawgatog's site here.