Support set operations in regular expression character classes

Feature or enhancement

Implement the set operations of Unicode Technical Standard #18 RL1.3 in re character classes, together with nested sets.

gh-74534 added FutureWarnings in Python 3.7 for the ambiguous constructs (--, &&, ~~, ||, and a leading [) as preparation for this syntax; this issue turns them into operators.

Two character sets are combined by an operator, where an operand may be a nested set in brackets:

• [A--B] — difference: a character in A but not in B.
• [A&&B] — intersection: a character in both A and B.
• [A||B] — union: a character in A or B.
• [A~~B] — symmetric difference: a character in A or B but not both.

Operators have no precedence and apply left to right; nested sets are used to group. A leading ^ complements the whole result.

For example, [a-z--[aeiou]] matches an ASCII lowercase consonant and [\w&&[a-z]] matches an ASCII lowercase letter.

Linked PRs

• gh-152153
• gh-152154
• gh-152214
• gh-153022

首先阅读 Python 的 re 字符类处理方式以及 Unicode Technical Standard #18 RL1.3。根据列出的差集、交集、并集、对称差、嵌套、从左到右处理和补集示例确认完成情况。