I recently did something similar, but different: gave Claude some code examples of a Rust-like language, it wrote a recursive descent parser for me. That was a one-shot, though it's a very simple language.
After more features were added, I decided I wanted BNF for it, so it went and wrote it all out correctly, after the fact, from the parser implementation.
Like three or four. very simple language: main function whos value is the error code, functions of one argument returning one value, only ints, basic control flow and math.
If I wrote down the prompts, I'd share them, but I didn't.
Please ignore the large amount of llm bullshit in here, since it was private while I did this, I wasn't really worried about how annoying and slightly wrong the README etc was. HEAD is better in that regard.
After more features were added, I decided I wanted BNF for it, so it went and wrote it all out correctly, after the fact, from the parser implementation.