It's not able to parse CFG grammers due to it being a stack machine so it can do backreferencing. I believe it can parse CFGs due to its ability to do "delayed regexes" - the (??{ }) construct. 5.004 for instance also uses a stack to parse regexes, but there's no way you can match balanced parens with a 5.004 regex. But you can make a CFG to match balanced parens.

Abigail

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