That makes sense. I think the easiest solution would to make it only run on tags starting with
Fanfiction. (with the dot). Having a bit of trouble with the regex though.
Code:
program:
if $$#fanficcat then
re(sublist($tags, 0, 1, ','), 'Fanfiction\.', 'Fanfiction/')
fi
didn't work (and it's a bit clumsy anyways)