User can regenerate completely so long as they are not instantly killed by an attack (i.e. disintegration, incineration, one-hit kill). For some users, this involves that as long as the "core" is undamaged, they can regenerate. Because the cells and telomere lengths are constantly regenerating and rejuvenating, they do not age, recover any damaged body parts, and sustenance intake required is drastically reduced, to the point user is close to true immortality. Even complex parts such as damaged brain cells and nerves can be repaired, effectively keeping the mind intact and making decapitation more difficult as the user can simply reattach their heads to the neck, sealing the cut.
User are practically immune to drugs, disease, and all harmful foreign substances, and will be forever in their optimal health and physical prime. They are immune, or at least highly resistant, to telepathic attacks, since the regenerative power restores brain cells to its unaltered state, blocking one's mind from any attacks or invasive attempts.
For tumorous cells, the regeneration power may consider it as a form of disease, and repair it to the healthy and optimal state so it does not grow along with the regeneration. For some users, however, it may consider the cells as part of the body, and actually regenerate these sick cells, causing them to grow painfully instead.
Ajins (Ajin: Demi-Human) possess such potent regenerative abilities that they can come back from the dead.
Vlad the Bad (Aria the Scarlet Ammo) can instantly regenerate from absolutely any form of damage unless the 4 demon entrails marks are destroyed at the exact same time.
Tenzen Yakushiji (Basilisk) having his symbiote "eat" away his wounds and restoring any ravages of time or battle, even reattaching his head by sealing the cut. Only complete incineration or neutralizing his ability can kill him permanently.
10 years after Tenzen's death, Joujin (Basilisk) gained the symbiote that was Tenzen's spirit, "eating" away any wounds and aging the same way Tenzen's symbiote did.
Teitoku Kakine (A Certain Magical Index) can regenerate himself with his Dark Matter, even if his entire body is destroyed, as long as there is Dark Matter in existence, as they contain his essence.
Sōsuke Aizen (Bleach) will regenerate from any injuries near instantly, no matter how severe, as part of the Hōgyoku's will to protect its master from any harm; Aizen is immortal and indestructible as a result.
As long as Genji Ikusabe (Buso Renkin) wields his Gekisen, he can regenerate even from being blown to bits. However, the process is automatic, so anything caught in between his regenerating body will cause it to fail.
Priscilla (Claymore) possesses regeneration of such a powerful level that it is more compared to creation, as she can rapidly regenerate from anything that is not instantly fatal, due to her infinite energy reservoir.
The One Being Sought (Code:Breaker) using Cell Regeneration to activate his cell's recovery ability, healing his wounds and recovering lost limbs and even his heart so long as nothing is lodged into the wound area.
Aoba Takatsu's subordinates (Code:Breaker) possess the ability Cell Stimulation, which causes their cells to grow and regenerate whenever they are attacked, gaining in size and power as long as not all of their cells are destroyed at once.
Garlic Jr. (Dragon Ball) succeeded in making a wish for immortality with the Dragon Balls, allowing him to regenerate from all bodily damage.
Cell (Dragon Ball) can regenerate from any injuries as long as the core in his brain remains undamaged.
Majin Boo (Dragon Ball) regenerating even after being blown to bits and ashes, so long as he has one molecule intact.
Peter Griffin (Family Guy) possesses regenerative powers so strong, that his severed hand regenerated an entire new body, into the evil Retep.
Being the very paragon of his own race, Count Vladislaus Dracula (Van Helsing) can regenerate at an advanced rate compared to that of regular Vampires.
Ryūzan Hayashi (Gintama) residing in No. 502, who can regenerate infinitely so long as the less-than-1 micrometer microchip is intact.
Utsuro (Gintama) harnessing the Altana energy of Earth to regenerate even from decapitation and cremation, and is considered to be immortal and ageless.
Alucard (Hellsing) allows himself to be near-liquefied at the beginning of several fights, just to completely demoralize his opponents before brutally killing them.
Kisame Hoshigaki (Naruto) regenerating from having his chest blown open instantly, and cannot tire out in battle due to the immense amounts of chakra Samehada absorbed for him.
Madara Uchiha (Naruto) absorbs the Shinju's true form into his own body, instantly regenerating from even a blown torso, and claiming he achieved complete immortality.
Kaguya Ōtsutsuki (Naruto) possesses immortality that she instantly regenerated her severed arm, including the sleeve of her robe of said arm.
Mukade (Naruto) regenerating at a very accelerated rate due to his modified puppet body in combination with the infinite Ryūmyaku chakra.
Genma Himuro (Ninja Scroll) has to put his severed limbs back together manually for regeneration, but he can do so for even decapitation and vertical bifurcation, achieving immortality.
Deoxys (Pokemon) can regenerate from even the most severe wounds as long as its core remains undamaged.
Using Battle Healing, Kirito (Sword Art Online) can rapidly regenerate himself during combat, healing wounds almost as quickly as they are inflicted upon him.
Tōta Konoe (UQ Holder) cannot regrow limbs unless they are completely destroyed, but otherwise is immortal and can reattach any of it, including his head.
After fusing with the tainted Shikon Jewel, Naraku's (InuYasha) regeneration powers were enhanced to the point he survived being torn apart by Inuyasha's Meidō Zangetsuha..
Road Kamelot's (D.Gray-man) real body in the real world has shown high regeneration ability.
Tech E. Coyote (Loonatics Unleashed) demonstrates.
Itsy Bitsy (Marvel Comics) is able to survive complete atomization and slowly regrow her own body.