[issue30349] Preparation for advanced set syntax in regular expressions

[Serhiy Storchaka]

Serhiy Storchaka <storchaka+cpython at gmail.com> added the comment:

Made a warning for '[' be emitted only at the start of a set. This significantly decrease the breakage of other code. I think we can get around without implicit union of nested sets, like in [_[0-9][:Latin:]]. This can be written as [_||[0-9]||[:Latin:]].

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<https://bugs.python.org/issue30349>
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