That is why the syntax in Rainbow is pretty similar.

The main problem had to do with Javascript regex not fully supporting everything that most languages support such as negative/positive lookbehinds and (.*)

1. TM syntaxes are complicated. There's lots of undocumented features that bundles find ways to use in crucial places.

2. TM is slow. You don't see this so much because it's written in C++ and there's many clever optimizations (see point 1). But it's difficult to get around the fact that a syntax is hundreds of regexes that have to be run over the whole document in a pathologically slow manner. I hate to think what it would be like in JS.

3. Many TM language definitions are dependent on features of the Oniguruma regex engine, which is strictly more powerful than any JS one. So you basically have to compile Oniguruma (no mean feat) ...with emscripten.