[michaelrpeskin]

Recently I did something similar to this on a small part of a project that has built up organically over a couple of years. One developer was getting the output of one program (that we didn't control) into one of our systems. So he wrote a python parser to read the output and put it in a structured format for us to read. Over the years edge cases and options kept creeping in, small features were added one at a time and it kept working.

We had to migrate to a new customer who didn't want the python middle layer, they wanted our software (Java) to read the output directly (don't argue on why this was necessary, just a requirement from a big customer). Since the python script grew up over time by many developers with small changes for new features, it was nearly impossible to understand exactly what it was doing.

I took only the parser code gave it to Opus 4.7 and said "This python code parses an undocumented file format. All of our tests pass and this code is the spec for the file format. Read the python code and create me a BNF document that describes what it parses." I took the BNF and started a new session and said "write me a parser in Java that parses the grammar in this BNF." It got it in one shot. Then all I needed to do is take what was parsed and pipe it into our existing software that took the structured format the python created.

Most of what I do is brownfield work and I really can't put our whole code base into an LLM because of IP issues, but for little things like this, is saves me weeks of work.