TextMate uses the oniguruma syntax for regular expressions.
TM4E currently uses jruby/joni as onigurama implementation. Unfortunately joni does not implement the the full onigurama syntax, e.g. variable length look-behind. This means that not all TextMate language files can be parsed with TM4E and loading fails with invalid pattern in look-behind. Since the goal of the joni library is to emulate Ruby's behavior and Ruby does not support variable length look-behinds, joni won't support them anytime soon.
vscode uses the original oniguruma C library from https://github.com/kkos/oniguruma/
It might make sense to investigate if we can do the same, e.g. using kkos/oniguruma via https://github.com/jnr/jnr-ffi/