[MrManatee]

Tree-sitter is the library that Atom uses to parse as you type. It's incremental: if you just add one character, you don't have to parse the entire file from scratch. And it's robust: it tries to provide useful results even if there are syntax errors.

If you're more interested in the theoretical side of how it works, there are some talks and articles that cover it.

https://tree-sitter.github.io/tree-sitter/