The parser should give a warning if a regexp matches the empty string, however it's no longer doing that. Looking at the code this is fairly obvious, since we pass an AFPData to checkRegexMatchesEmpty, but that method doesn't parse the regexp correctly (i.e. slashes are not escaped, and delimiters aren't added), so it never matches.
Customize query in gerrit
|mediawiki/extensions/AbuseFilter||master||+22 -15||Pass a valid regexp to preg_match in checkRegexMatchesEmpty|