[issue12759] "(?P=)" input for Tools/scripts/redemo.py raises unnhandled exception

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

> 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.

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nosy: +serhiy.storchaka
stage: patch review -> needs patch

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<http://bugs.python.org/issue12759>
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