In episode 16 of Full Metal Alchemist, That Which is Lost, a harmonica piece identical to Regret is played near the end of the episode while Ed is having a discussion with the Ishbalin soldier.

The piece begins at around 18:10:

http://www.animefreak.tv/content/fullmetal-alchemist-episode-16

Anyone know the name?

Any help would be appreciated. :)