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

Start by reading Python's re character-class handling and Unicode Technical Standard #18 RL1.3. Confirm completion against the listed difference, intersection, union, symmetric-difference, nesting, left-to-right, and complement examples.