[ownedbycats]

program:

# Returns 'Fanfiction/Fandom' for fanfics
	if $$#fanficcat then
		re(sublist($tags, 0, 1, ','), '\.', '/')

# Returns 'PDF' for PDFs
	elif $formats == 'PDF' then
		'PDF'

# Returns topmost tag for all other books
	else
		re(sublist($tags, 0, 1, ','), '^(.*?)($|\..*$)', '\1')

	fi

this works!