The problem with answering this question is capturing the notion of "unbounded" in an actual implementation. For example, the regex `/(.*)\1/` will capture the language $L = \\{ ww | w \in \Sigma^*\\}$ which is <strike>a CFL</strike> much more powerful. In practice there might be limits on the stack used (i.e maybe $w$ can't be longer than some large number $K$), which would effectively turn the language into $L_K = \\{ ww | w \in \Sigma^*, \mid w \mid \le K\\}$, which for any fixed $K$ is a regular expression again. 

But in principle, regexps as specified are more powerful than regular languages, as [this related question][1] discusses in much more detail (with a nifty example as well). 


  [1]: https://cstheory.stackexchange.com/questions/448/regular-expressions-arent