Writing a JSON Parser From Scratch: Lexer, Recursive Descent, and the Cases You Forget
I’ve been reading JSON for years without ever writing the code that parses it. That’s a strange gap: it’s a format we touch most days, and the one whose internals I’d thought about least. So I sat down one evening and wrote one from scratch in Python, no json.loads, no regex shortcuts, just the spec and a blank file. The structure is the textbook one: a lexer that turns characters into tokens, and a recursive-descent parser that turns tokens into a tree. What I want to write about is less the structure itself (that part is well-trodden) and more the places where the spec is sharper than it looks, and where a reasonable-seeming shortcut quietly produces a parser that accepts invalid input. ...