The filter matches using both the URL and the title, case-insensitively.
test will simply search for test. !test will exclude anything containing test.
test, !b will include anything containing test that does not contain b.
test, b will search for anything containing test or b. There is no way to do 'and' instead of 'or'.
https://workplace, https://stackoverflow, !test will show only posts from workplace and stackoverflow, that do not have 'test' in the title.
!https://workplace,!https://stackoverflow will show everything except workplace and stackoverflow.
You cannot use escape characters.
Exclamation points that are not at the start, do not negate the search. E.g. no! is not a negative filter.
A good way to match sites is by matching just a little bit around the site name. E.g. https://security.stackexchange.com/question/x can be matched using /security. and mathoverflow using /mathoverflow..
A set of filters that I like is this.

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Author: Luc Gommans | @lucgommans