The rules are a Domain Specific Language based on the Xbase language and it bares the most resemblance to the Xtend language with some missing features (e.g. Xtend supports classes and arrays but the DSL does not). Your best bet is to look at the Xtend documentation here for the syntax of expressions (e.g. for loops, case statements, etc) and the Rule’s wiki page here for how rules files themselves are structured.
As for how it is implemented internally I can’t say. But if there are grammar files they will all be in the GitHub repo.