I had to write mid size custom markdown parser?/lexer? with many various business requirements or things like attaching additional informations that'd allow frontend to display completion popup menu

I wrote it just as "just" step by step algorithm that transforms e.g 500LoC into flat tree of parsed objects

I thought about learning formal grammar theory, but I couldnt see how it'd help me because at the end everything worked fine. It just needed writing a lot of tests.