Of that, I do not know.   What I do know is that I have asked impossible things of that module and it did every single thing that I asked with grace and style.   (Parsing hundreds of SAS® programs and TWS® schedule files and Korn shell scripts ... oh my!)

I sincerely think that the problem lies in your grammar and that a trace output will reveal the answer you seek.   I have scratched my head in a very similar fashion many, many a day.