[Bug] Monarch Tokenizer negative "lookbehind, lookahead" not working

[fdrag]

Reproducible in vscode.dev or in VS Code Desktop?

• [X] Not reproducible in vscode.dev or VS Code Desktop

Reproducible in the monaco editor playground?

• [ ] Not reproducible in the monaco editor playground

Monaco Editor Playground Code

return {
  tokenizer: {
    root: [
      [/\b(?<=var)\s\w+/, 'keyword']
    ]
  },
};

Actual Behavior

lookahead not woring

Expected Behavior

Additional Context

highlight words that doesnt start with var + whitespace

[14glwu]

the same problem

[medihack]

I guess this depends on browser support of lookbehind which is not perfect https://caniuse.com/js-regexp-lookbehind