Regen (short for Rengenration and also known as HP Regen and FP Regen) is a special status effect that appears in the Paper Mario series.
In this game, there are two types of Regen that appear. As stated above, it being HP Regen and FP Regen.
HP Regen is mainly associated with HP being restored over time. There are numerous of ways to obtain the HP Regen status. The best way is to use a Slow Shroom that gradually restores 2 HP for the course of 10 turns. Certain dishes can also gradually restore HP over time but is usually different each time (such as the Meteor Meal restoring 2 HP for 3 turns but also restores 7 HP on the spot and the Couple's Cake restores 2 HP for 10 turns).Badges such as Happy Heart and Happy Heart P also does HP Regen but only restores 1 HP for the entire battle.
FP Regen works similar to HP Regen but only for FP. The Gradual Syrup is a good item to use to achieve the FP Regen status. It restores 2 FP for the next 10 turns. The Lucky Start badge may grant the FP Regen status as also the Happy Flower badge.