Easier way to have many sub-session group filters

I currently have a massive set of IIF statements to filter 1 group into several sub-groups here is an example:

IIF (GuestMachineName LIKE '*A-*', 'A',IIF (GuestMachineName LIKE '*B-*', 'B',IIF (GuestMachineName LIKE '*C-*', 'C','OTHER')))

This shows a tree like:

GROUP

-> A

--> A-1

--> A-2

-> B

--> B-1

--> B-2

-> C

--> C-1

--> C-2

-> OTHER

--> D-1

--> E-1

Is there an easier way to do this? If not this would be my request