But Perl regexes have backreferences. Which makes them more powerful than finite automata. Note that your first quote could also be used to prove you cannot accept strings that are a composite number. But /^(..+)\1+$/ proves that you can.

Note also that Perl 5.10 regexes have named rules, allowing recursion. Which means, IMO, that Perl regexes are at least as powerful as PDA (push down automata). Which means they should be able to accept any context insensitive language.

The classical example is that with a "regular expression" you cannot match strings for the form aⁿbⁿ, but that you need a PDA. /^(a(?1)?b)$/ matches such strings.