Problem deriving artist from filename

Support
3

Using Mp3tag's special mask features you do not have to do complicated math.

Have a look at the filename structure.
Choral Fantasy -- Op. 80_Otto Klemperer_New Philharmonia Orchestra_Daniel Barenboim (piano), John Alldis Choir.flac
part1 -- part2_part3_part4_part5

For parts you do not want to extract you simply use the dummy placeholder, otherwise use the appropriate named placeholders.
If you want to extract only the artist part you can write such a mask:
%dummy% -- %dummy%%dummy%%dummy%_%artist%

  1. You can use converter.

Convert | Filename - Tag | ALT+2
Select format string
Examplestring: Choral Fantasy -- Op. 80_Otto Klemperer_New Philharmonia Orchestra_Daniel Barenboim (piano), John Alldis Choir.flac
Formatstring: %dummy% -- %dummy%%dummy%%dummy%_%artist%
Preview
artist | Daniel Barenboim (piano), John Alldis Choir
  1. You can use action.
    Actiontype 7: Import tag fields (guess values)
    Source format: %filename%
    Guessing pattern: %dummy%÷--÷%dummy%%dummy%%dummy%%artist%
    Note: Replace one special ÷ character with one space character.

DD.20090910.0606.CEST