For the sake of discussion, here's what the author says: http://rosie-lang.org/blog/2018/02/25/why-rpl.html#why-not-u...

I guess pest is comparable then, but wasn't mature when the author started work on Rosie?

(I'd be curious for a proper comparison, but I'm not really knowledgeable in this area -- I had no idea there were so many alternatives to regex: https://en.wikipedia.org/wiki/Comparison_of_parser_generator...)