You can go pretty deep once you get context free grammars. For example, I'm using torch-grammar (but outlines should be able to do the same thing once CFG support is merged) to not just restrict the format of a generation to a DSL's syntax, but to restrict the keys it updates to valid keys in a known set.

e.g.:

    int_key ::= DQUO ("f" ("e" ("atured-" ("b" ("log." ("p" ("ost_limit" | "a" ...

Obviously, yeah, it doesn't "understand" the content, but that's what the LLM is for. It's remarkable how plausible the generations you can get out of random noise are with a sufficiently-restrictive grammar. Bolting that onto a well-trained LLM is pretty powerful.