This preprocessor is based on JavaCC21's preprocessor. Whereas that is based on strict adherence to C#'s preprocessor (symbols are Boolean values only), this version expands the functionality so that you could add symbols with string values, perhaps by passing -D<sym1>=<value1> -Dsym2=value2 on the command line. Only simple identifiers are supported as values (e.g. no spaces or punctuation, no quoted strings) in order to support constructs like #if CODELANG == python in preprocessed files. This is to potentially facilitate support for multiple code generation languages in JavaCC source files:

#if CODELANG == python
Some Python code here
#elif CODELANG == csharp
Some C# code here
#else
Some Java code here
#endif

This code is currently at a proof-of-concept stage; more work may be needed, and more tests are definitely needed. Jython is used for running the (JUnit-style) tests, so you may want to download it if you want to run the tests. I use the standalone Jython jar and a simple shell script:

#!/usr/bin/sh
java -jar $HOME/.local/bin/jython.jar $*

Jython is used as it's quicker (for me - YMMV) to write tests in than Java would be.