Comment by jibal

Actually TFA is explicit about this:

> Regular expressions in the formal grammar sense can (pretty much by definition) only parse regular grammars and nothing more.

> But when programmers talk about “regular expressions” they aren’t talking about formal grammars. They are talking about the regular expression derivative which their language implements. And those regex implementations are only very slightly related to the original notion of regularity.

> Any modern regex flavor can match a lot more than just regular languages. How much exactly, that’s what the rest of the article is about.