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About the test cases, they're missing indeed. I can write
some while applying the patch.

About being experimental, IIRC, it is listed like
experimental in the Perl documentation for several years,
and will probably stay like this forever. :-) Anyway, IMO
this shouldn't affect our evaluation of the importance of
that feature for Python's sre.

About semantic restriction, do you mean check if the
backreference is lesser than the current group? Should be
doable. OTOH, I don't understand your example. In
"(X)|(?(1)Y)", there's no sense in using (?(1), as it will
always be false.