Am closing the feature request because it has very little chance of making it into the core distribution without being a third-party module first.  The existing implementation is already somewhat difficult to maintain and we don't want to make that worse.  Also, the new functionality is of interest to only a small subset of the re module users.

If you do write it, respect the GPL license on the SML code and take some care to make sure you can license your new code in any way you see fit.