Below is a saying from this article.
Regular expressions sit just beneath context-free grammars in descriptive power: you could rewrite any regular expression into a grammar that represents the strings matched by the expression. But, the reverse is not true: not every grammar can be converted into an equivalent regular expression.
Can anyone tell me if the saying is correct and why? Per my understanding, they should have same power and the differernce is a context-free grammar has recursive definition.