Regular expressions: recursive patterns and callouts

[Carlos]

Hi!
I think two extensions to re could be really powerful in aiding to
match non regular strings -for example those containing parens nested
up to an arbitrary depth- but without a significative performance loss,
although I'm not at all sure at this last point. One of these features
is re nesting; I don't mean nesting some strings and then compiling the
composed re, but to include -possibly recursive- references to other
re's. The second feature is the ability to add callout points in a re,
so external functions would be called when the matcher reaches those
points to validate the current matched prefix or even to do some
arbitrary further matching of its own, advancing the pattern position.
pcre implements similar features with its (?CN) callout and (?N)
recursive patterns mechanisms.
Are the above or equivalent features planned for future python
releases?
Do you know of some extension package which provide them or at least
pcre compatible res?
Thank you in advance,
Carlos