Is it possible to capture whole fields for lookahead and lookbehind RegEx in keygenerator?

I tried some stuff, but it does not work.

e.g.

(?<=[KEYWORDS])123

Translation: Match 123 only, if there is no data within the keywords field.

This is the resulting entry from generating a key with the above pattern:

@Article{<123,
  title = {333},
}

The citationkey turns out to be <123.

So this clearly is not the result i was expecting.

Is there a special RegEx that users can use and that would allow me to do this, or would Jabref need to add some code that would make this work first?

Documentation about regular expressions for Java: Pattern (Java Platform SE 8 )
Documentation about Conditional RegEx: Conditional Regular Expressions—from 101 to Advanced