Backreferences are neither enough, nor necessary to make up the difference between languages that are recognized by FAs ("regular expressions") and those that are recognized by PDAs ("context-free grammars").

As I pointed out, a classical context-free language that isn't a regular expression is anbn. You cannot match that with 5.8 regular expressions (assuming no jumping back to the interpreter using (?{}) or (??{})), proving that backreferences aren't enough. But in 5.10, you can write /^(a(?1)b)$/, which doesn't use backreferences. It does use recursion, but recursion is the difference between an FA and a PDA.

Now, I think that due to backreferences, Perl regular expressions are more powerful than context-free grammars, but I cannot think of a language that is matched by a Perl regular expression, and not by a context-free grammar. And I don't think Perl regular expressions are as powerful as context sensitive grammars (which requires a TM). For instance, I don't think there's a Perl regular expression to match anbncn.

In reply to Re^3: check for square-number with a regex by JavaFan
in thread check for square-number with a regex by Ratazong

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