This question is mostly about grammar, not about code:
In my mind, grammar reviews do not belong on Code Review. Does Meta agree?
Questions about grammar probably are off-topic; for instance, "Does this CSS code have correct syntax?" and "Where is the syntax error in this CSS code?" and "How do I express this in CSS?" and "Is my new language as cool as CSS?" are not about working code or its quality. The post you linked is not for a review of CSS (or any other language) grammar, however; it's a request for a review of the quality of a PEG source.
It was also possible to achieve the same task with regular expressions in JS or any other language that supports them. Again, as JS code, unquestionably on-topic, but next to impossible to read and thus review.
Just my take on it; if meta disagrees, I will be happy to replace the PEG with one of those more preferable forms.