Run:
python <PYTHON_PATH>/Tools/scripts/redemo.py
Enter "(?P=)" in entry field.
See unhandled exception occures.
There is special text field in example for such cases. It should be used to show error messages, not a console.

I can reproduce in 3.3 (the file has been moved to Tools/demo/redemo.py).  The Tk application does not crash but there is a traceback.  Would you like to work on a patch?  If so, there are good guidelines in the devguide.

I would like to make a patch.

Great!  Feel free to ask any questions here or through the core-mentorship mailing list.  Remember to read the devguide and work from a Mercurial clone of Python 3.2.  Thanks!

I am working on the patch. The fix is ready. Will be delivered shortly.
It is also broken when I specify (?P<>) pattern

The problem with the above expression is that it does not specify a group name. The expression itself is not right. So, it blows up when it tries to parse the name. I put the check for the empty name before any processing is done.

Thanks for the patch!
The patch should also include tests for this case in Lib/test/test_re.py

Updating the patch with the test. The test checks that the re exception is thrown in case of the (?P=) and (?P<>) expressions.
It used to raise an Index exception. My only question is, the exception we raise now says, there invalid characters in the group name, whether in our case the group name is just empty.

> My only question is, the exception we raise now says, there invalid characters in the group name, whether in our case the group name is just empty.

Good point. It would be better if you move the test outside isname() function and immediately raise an error with an appropriate message.

Here is an updated patch. More appropriate error message used, many new tests added.

New changeset 5b5c2c3b2268 by Ezio Melotti in branch '2.7':
#12759: sre_parse now raises a proper error when the name of the group is missing.  Initial patch by Serhiy Storchaka.
http://hg.python.org/cpython/rev/5b5c2c3b2268

New changeset 29983dd7191e by Ezio Melotti in branch '3.2':
#12759: sre_parse now raises a proper error when the name of the group is missing.  Initial patch by Serhiy Storchaka.
http://hg.python.org/cpython/rev/29983dd7191e

New changeset 28be586287c8 by Ezio Melotti in branch '3.3':
#12759: merge with 3.2.
http://hg.python.org/cpython/rev/28be586287c8

New changeset 3f4fbad75c99 by Ezio Melotti in branch 'default':
#12759: merge with 3.3.
http://hg.python.org/cpython/rev/3f4fbad75c99

I updated the patch to use 'missing group name' instead of 'bad group name', and added a couple more tests.
This also fixes the issue in redemo.py.
Thanks Serhiy and Masha for the patches!