Does a Troll benefit from its Regeneration trait if it is surprised?

Question

The rules of surprise in D&D 5e state:

If you're surprised, you can't move or take an action on your first turn of the combat, and you can't take a reaction until that turn ends.

A troll has the Regeneration trait, described as follows:

The troll regains 10 hit points at the start of its turn. If the troll takes acid or fire damage, this trait doesn't function at the start of the troll's next turn. The troll dies only if it starts its turn with 0 hit points and doesn't regenerate.

I know that monsters can't take legendary actions while surprised, as clarified in the Monster Manual errata.

However, traits aren't actions, and the troll's Regeneration trait in particular doesn't require an action, a bonus action, a reaction, or anything; it simply happens at the start of the troll's turn as a passive ability.

And since surprised creatures still take a turn during the first round of combat, even if they can't move or take actions (or bonus actions, as the last sentence on PHB p. 189 clarifies), this would suggest that the troll still regains 10 HP at the start of a turn in which it can't move or take actions.

Is this an accurate interpretation of the rules, or have I missed something?

Answer 1

Your interpretation is accurate.

Though in many groups this is not exactly how it is played out (some tend to allow a surprise round to take place sort of out-of-order before rolling initiative for everyone else), the rules as written make it clear that at the beginning of a combat, all creatures should roll for initiative and have a place in the turn order, even if they are surprised and therefore will not actively do anything in the first round; thus the troll gets a turn during the surprise round on which it takes no actions and does not move. If it has already taken damage, that damage will start regenerating, since as you correctly note, regeneration is a passive ability, not an action or reaction of any kind that the troll must specifically take.