regeneration effect to the attacker or the victim?
for either one you should open the page of your tool, go to "Triggers" on the bottom, and then make a procedure for "When living entity is hit with tool" that applies regeneration
if it's the victim, leave it as Event/target entity, if it's the attacker replace Event/target entity with Source entity which can be found in the Minecraft components tab
regeneration effect to the attacker or the victim?
for either one you should open the page of your tool, go to "Triggers" on the bottom, and then make a procedure for "When living entity is hit with tool" that applies regeneration
if it's the victim, leave it as Event/target entity, if it's the attacker replace Event/target entity with Source entity which can be found in the Minecraft components tab