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Yes, Chromium.

No, there's actually a C++ backend under there that implements the editor, if I recall correctly.

A C++ backend, aka Electron.

I think you're confusing Atom and VS Code.

VSCode uses the same c++ based regex parser for syntax highlighting as TextMate, Sublime, Atom, etc

For anyone interested, this is the Oniguruma [0] regex library written by K. Kosako. It's also used in Ruby and PHP.