Yup. In fact Perl 5.6 does a version of this. However doing it with a run-time cache has several serious disadvantages.

First of all it is much slower because you are always having to check, add information. With 5.6 the RE is labelled simpler or complex, and profiled at run-time, only having caching turned on if it seems slow.

Secondly there are a variety of situations in which you have to throw away the cache. At least one of the RE bugs in 5.6.0 is that it was not smart enough in doing this and got it wrong in a few cases. It appears that with my approach it takes less work to figure out when you can and cannot optimize.

Thirdly when you do the work at compilation, you get multiple speed wins for the price of one. For instance this one also covers trieing the RE, which is something that run-time caching does not do.

And so on.