Spread the light, my child.
Henzell says...
  1. To calculate the rate at which you regain hp, see [2. For the artifact property, see [3. For the mutation, see [4. For god abilities, see [5. For the removed spell, see [6.
  2. The base regeneration rate is 20+mhp/6. This is added to any bonuses (from equipment, mutations, etc). This total accumulates every turn, and every 100 points of "regeneration rate" gives you 1 hp. Prior to 0.30, base regen rate was mhp/3 for mhp up to 60, after which it was 10+mhp/6.
  3. One regen pip is worth 80 regen rate, or 0.8 hp/turn (on average). This is a tremendous amount of HP: To match this naturally, your mhp must be 420. Equipment granting regen+ must be attuned to you and will not grant hp until you have had it equipped at full health.
  4. Each level of the mutation is worth one pip (80 regen rate or 0.8 hp/turn). The demonspawn Powered By Death mutation grants 100 regen rate (1 hp/turn) for each activation of the mutation.
  5. Trog's Hand grants 100 regeneration rate (1 hp/turn). Jiyva's regeneration is worth 1 pip (80 rr, 0.8 hp/turn) at 1* piety, and 2 pips at 6*, scaling linearly.
  6. Prior to 0.25, was a L3 Charms/Necro spell that granted the same regeneration as trog's hand (100 rr, or 1 hp/turn) for a power-dependent duration. Removed for being a no-brainer decision for every character to memorize, cast, and have up all the time.
  7. If a character cannot regenerate passively (this means DD or bloodless Vp or any character with slow healing 3), items or spells that increase regeneration rate won't help. Trog's Hand and potions of ambrosia will, though!
  8. Prior to the removal of food, all sources of regeneration (Other than Jiyva, whose regeneration was different at that time anyway) increased the rate at which your hunger increased.
  9. Prior to 0.28, one pip of regen was worth 0.4 hp/turn rather than 0.8 hp/turn, and Trog's hand and Powered by Death were 2 pips rather than a flat 1 hp/turn.
  10. <crate_> regen is a spell you cast if you are not at full hp
    11. More...

Information based on master branch and the great LearnDB.
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