Inspired by this answer and subsequent comments, I am curious if a PC can opt to take only part of the benefits of a regenerate spell.
The regenerate spell description states:
The target's severed body members (fingers, legs, tails, and so on), if any, are restored after 2 minutes. If you have the severed part and hold it to the stump, the spell instantaneously causes the limb to knit to the stump.
However, using the example from the linked question, say a PC wanted their severed tail to regenerate, but not their self-mutilated horns – would the PC be able to partially reject the regenerate spell, thereby allowing their tail to regenerate and not their horns?
Best Answer
No.
Regenerate says (PHB, p.271, emphasis mine):
The phrase "natural healing ability" implies that anyone subject to the spell would only have as much control over the healing as they would over their body's natural healing -- that is, not at all. Also note that the spell doesn't say "a willing creature", and there's no way to resist the spell. So based on the spell description, regeneration is irresistible and absolute.
As a DM, I might allow any of the following if the players expressed these intentions:
The caster of regeneration might be able to exclude some body parts from regenerating.
The subject of regeneration might be able to make a Constitution or Wisdom save to reject the entire spell.
A character with some control over their own natural healing — a monk being the most obvious example — might be able to make a Wisdom or Constitution check (not save) to control how the regeneration proceeds.