Which languages do Perl-compatible regular expressions recognize?
I've also found this blog post extremely interesting http://nikic.github.io/2012/06/15/The-true-power-of-regular-expressions.html. It provides the very same proof I gave before about the fact that regexps recognizes the CFLs (by rewriting the grammar through DEFINE blocks), and even some CSLs (like the language of repeated strings); it builds on that and goes on, giving a proof that regexps with backreferences are NP-hard (by reducing 3-SAT to a regexp).