I've found build.bat --regen unreliable myself. I went over it with Steve and one issue that came up is that it uses the python.exe that is built to run the code generation scripts; OTOH on Linux/Mac these scripts are run using a suitable pre-existing Python binary, e.g. a recent enough system Python. (However, the frozen files require a binary (_freeze_importlib).

It will take some time to refactor things so that this is more usable.

Perhaps instead of (or in addition to) doing everything via .vcxproj files, we could have a simple(r) regen.bat script that runs all the regen scripts (or at least the non-freeze ones) using the system Python?

FWIW my own issues weren't with the parser, they were with regenerating opcode.h and opcode_targets.h.