Works perfectly. Thank you so much--this will save a ton of time.
Here is a quick guide for those wanting to use these expressions in the future to separate out featured Artists from the Name field:
1) Tools -> Options -> Library & Folders -> Manage Library Fields -> Add New Field Name your new field Featuring and select the Calculated data radio button. Enter the expression:
replace(replace(replace(replace(replace(regex([Name],/#(.+)\s+\(*(featuring|feat|ft)\.*\s+([^\)]+)\)*$#/,-1,0)[R3],/,,;),&,;),and,;),/ ;,;),/; ,;)
Click OK
2) Go back into Manage Library Fields -> Add New Field Name your new field Name Cleaned and select the Calculated data radio button. Enter the expression:
if(regex([Name],/#(.+)\s+\(*(featuring|feat|ft)\.*\s+([^\)]+)\)*$#/,0,0),[R1],[Name])
Click OK -> OK
3) Go to your Panes, Categories, or File List views or the Tag Window
4) Ensure the Album Artist field is populated with the main Artist you would like responsible for the Album. If it is not, you can correct this by entering =[Artist] or =[Album Artist (Auto)] into the Album Artist field.
5) In the Artist field type =[Artist]; [Featuring]
6) In the Name field type =[Name Cleaned]
A couple notes:
1) As always, when editing tags or using new expressions, start by going to File -> Library -> Back Up Library to back-up your Library before beginning. Also, test on a small set of files first to make sure it is behaving as you expect.
2) The nice part about this system is you can use it to batch rename files once you know it is behaving correctly (ie do all Tracks in an Album at once) since it will just default to the original Name and Artist if no Featuring Artist is in the Name field.
3) If a Featuring Artist has "&" or "and" in their name (Ex Simon & Garfunkel or Jason Isbell and The 400 Unit), this expression will separate that Featuring Artist into two separate Artists (Ex Simon; Garfunkel) so you must be careful in these cases. It assumes that these will be the exception, so you will need to correct them manually.