you can make a filter over fanfiction as follow:
needle => contain needle
~needleA ~needleB => is needleA or needleB
-needle => doesn’t contain needle
char:harry => have character named harry
pair:harry/hermione => have a pairing of harry with hermione
kudo:>1000 => have more than 1000 kudos
chapter:1 => has exactly 1 chapter
bookmark:5 => has been bookmarked exactly 5 times
comment:>50 => have more than 50 comments
hit:>10000 => has more than 10000 hits
words:>3000 => have more than 3000 words
updated:>2months => have been updated before 2 months ago
is:complete => is complete
is:f/f => has lesbian relationship
is:m => has mature rating
fandom:firefly => is of the firefly universe
is:crossover => is a crossover
tag:searchtag => has a tag containing “searchtag”
lang:english => is in english