I remember seeing this on /r/PHP, and one of the top comments there was about it using Regex instead of parsing it like a language.

However, I also recall that it's thanks to using regex that it works so quickly. So I figured I'd get this argument out of the way before someone else brought it up.

From having tilted at this windmill a little myself, I think:

1. It's tricky enough to handle correctly all the under-specified corner cases of basic markdown -- not to mention the popular extensions to it. The cognitive load of doing it with complex regexps gets heavy, quickly.

2. I'm incredibly impressed with all the work that John MacFarlane has put into the problem, for example in [Pandoc] and [Cheapskate].

[Pandoc]: https://github.com/jgm/pandoc

[Cheapskate]: https://github.com/jgm/cheapskate