I am very skeptical. This state-machine approach seems much more like an implementation than a specification. Having a reference implementation could certainly be a good thing, but this doesn't even look like something one could run and test against.

The declarative form of Comment, above, is wonderfully concise and clear when compared to these several lines of imperative, update-this/goto-there style alternative. You can see in your head what it should match without mentally simulating these specific instructions against some imagined parser state.

There certainly can be a host of terrible issues with BNF-style grammars. When they're just used as a notation to write down a bunch of rules, with no regard to actually implementing these rules, the result can be a sprawling and terribly ambiguous mess. For instance, this[1] is an abject disaster, chock full of ambiguity and senseless distinctions.

But if one is prepared to take the effort to really write a machine-readable grammar like this[2], the result is a straightforward, high-level, concise spec that can be compiled into an implementation to boot. What's not to like?

[1] http://sven.xtreme-eda.com/ [2] https://docs.python.org/3/reference/grammar.html