%discnumber% IS 1 AND NOT %_filename% MATCHES (\d+\-\d+)
It this one does not match for your filenames, please provide real existing file names from your collection.
It matches, thank you.
(I assume this is a follow-up question to the Assistance Needed to Rename Files More Efficiently thread?)
I am now knee-deep into it, so I am constantly trying to improve efficiency.