It seems to me that one should be able to detect which features are actually used in a regular expression and choose one of multiple different underlying implementations based on that.

Newer versions of Perl actually support a pluggable regex engine system, so you can use specific regex engines for specific tasks.

If you hook in libpcre direct it actually comes with its own JIT these days. http://sljit.sourceforge.net/pcre.html

Having the compiler available at runtime kinda helps too!