Most of the runtime needs in this space can be managed with circuit breakers without having to provide separate processes or memory spaces, be they temporal or resource based.

For example your regex, could be given 20ms to run and use a maximum of 8Mb of heap before it is interrupted. This can happen on a single thread and drop back to the caller if an exception is thrown. I'd love to see a language feature which defines the maximum stack and heap for a particular scope i.e. in C#:

   Breaker.Heap(8.MiB(), () => {
       Breaker.Time(200.Milliseconds(), () => {
           // risky operation
       });
   });

(we already do the time breaker, but not the heap)

Edit: correct calling convention.

Edit 2: add missing extension method brackets