Currently only PCRE-style regex is supported, which does not support negating the expression. You can try to use negative lookahead like /^(?!.*(good)).*$/ but that will match posts that lack “good” in either the body or the title, so unless it is in both, it will not be filtered out.

This is one of my biggest gripes I have with selfoss. We have an issue tracking more advanced filtering.

I have decided it to split the advanced filtering into a separate feature request in order not to block this to for selfoss 3.0.

Until then, you might be able to use something like http://feedrinse.com/index-old.php.