I already tried hard to figure out what "Regen" means during the whole year of FM2010. Now I took all my courage together to ask this probably super silly question: What is "Regen"? ;(
Regens are randomly generated players that are spawned for all clubs each year. The date they are created each year varies by country. As the game progresses real players inevitably get older and retire. They exist to keep the game populated.