[RPG] What creatures have the regeneration ability

You are reading this correctly. Trolls are notoriously difficult to kill, able to regenerate back from pretty much anything (including things like decapitation) in minutes.

This also becomes pretty clear if you read their lore block, and adventurers do well to prepare if they are going up against these beasts.

Keep in mind though, that a single point of fire or acid damage is enough to disable regeneration, so smashing the Troll down to 0 hp and then burning it with Alchemist's Fire or Acid will make sure it can't recover. Alternatively, dragging it into a campfire also works.

Once the Troll is disabled and there are no other monsters left, it's usually quite doable to deal with one.

I'm going to give a somewhat expansive answer here, because I think the topic of vorpal swords needs one.

tl;dr - Decapitated is a different status effect than Dead in 3.5e, and D&D in general; consult the monster stat block for more info.

A Vorpal weapon does not explicitly kill anything; the wording is important. As per the text, "Upon a roll of natural 20 (followed by a successful roll to confirm the critical hit), the weapon severs the opponent’s head (if it has one) from its body."

That is all it does. As the text goes on to explain in some examples, there is a very common resulting effect: "Most other creatures, however, die when their heads are cut off."

Creatures without a head are implicitly defined as unaffected in the text; "Some creatures, such as many aberrations and all oozes, have no heads."

Some creatures do not die from losing their head(s), and if the text is read as RAW, by default do not take any effect, negative or positive, from losing their head(s): "Others, such as golems and undead creatures other than vampires, are not affected by the loss of their heads."

A few other unmentioned creatures do not die from a vorpal effect, or at least not immediately. One example is given below.

So, the vorpal weapon doesn't "kill", per se. It decapitates. Death is a possible side effect. It comes down to the individual creature statblocks and the effects therein. For example, hydras with multiple heads can survive at least one vorpal attack, by RAW: "A hydra can be killed either by severing all its heads or by slaying its body."

http://www.d20srd.org/srd/magicItems/magicWeapons.htm#vorpal

http://www.d20srd.org/srd/monsters/hydra.htm

Edit: For a more specific answer, see Nepene Nep's post.