Yes, that's right, the last word of the composer. And as I've got it, it includes the space before that last word as well, which isn't ideal. But as my composer fields are formatted as "First Middle Last" it worked for my purpose, which was to put the composer's last name in the filename. It would fail in the case there was a suffix like "Johann Stauss Jr."
A regex to grab the first word should be even simpler and could key on the presence of a comma.