LLMs for structured input validation

You might want to try Instructor with llm_validator, where you can have natural language clauses to check against in a loop: https://python.useinstructor.com/blog/2023/10/23/good-llm-va... With Pydantic too of course!

What would be the advantage of using an LLM here? As opposed to parsing/validating the input using any other technique (of it's a well defined format, where you have e.g. a grammar).

Agree with this. Just use Pydantic.

Depends on how fuzzy your definition of template is. If it's really structured, a simpler algorithm might work better. Otherwise maybe try https://github.com/Unstructured-IO/unstructured