[DayCuts]

Wrote quite a lengthy/detailed response breaking down the problems with your attempts, the misunderstanding about how lookarounds work, and how to design a pattern that works but my browser crashed so you will just have to settle for the footnotes and research yourself to get a better understanding.

Pure PCRE solution:

(?im)^(?!.+-(publisher1|publisher2)$).+$

/subdir/subdir/.../Author1_-_Title1_(1234)-Publisher1
/subdir/subdir/.../Author2_-_Title2_(1234)-PublisherX
/subdir/subdir/.../Author2_-_Title2_(1234)-Publisher2

Other notes:

Originally Posted by bigstar

What might be more suited for what you desire is to use the Selective Transfer feature.

I agree with this suggestion, regex pattern matching was not designed for 'non-matching'. Although it can be done the internal processing is more expensive for anything other than use with single characters, as is the use of lookarounds, etc. While the above pattern should work in the Skip List if PCRE matching is now also possible within selective transfer rule sets I would highly suggest ditching the expensive 'non-match' style negative lookahead pattern and opting for a normal 'match' style pattern.

Originally Posted by bigstar

I've made a small change to the syntax prefix

Can I suggest you reinstate the colon as part of the prefix? There should be no circumstances in which somebody might try (or be able) to match 'rx:<space>...' as a literal (non regex) pattern, however there is the possibility of somebody trying to match 'rx<space>...'.

Originally Posted by bigstar

It took me some time to figure out the proper way to ignore case with PCRE, I am not 100% sure if this is correct.

rx (?i).*-(?!publisher1)

Your use of (?i) here is correct. Given that FlashFXP is a windows client and windows (and the users there of) mostly think is a case-insensitive manner it might be okay to make it case-insensitive by default. Just so long as the case-sensitive modifier can be used within the pattern. (?-i) would force case sensitivity.

Modifiers/switches can be used anywhere in a pattern. When a modifier is seen it is explicitly applied to the remainder of the pattern, or until switch by another modifier. The basic form of a modifier is (?[onswitches][-offswitches][:regex]). This support for :regex means you can do things like (?i)^x(?-i:Y)z to match any case form of xyz as long as Y is capitalized, where (?-i:Y) is equivalent to (?-i)Y(?i).

A great regex introductory tutorial can be found at Regular-Expressions.info - Regex Tutorial, Examples and Reference - Regexp Patterns