<regex>: Avoid generating empty groups when parsing ? quantifiers

[muellerj2]

Towards #5962. The parser currently generates empty groups when it represents zero-one quantifiers like ? by a _N_if node with an empty alternative, just as it did for empty alternatives in general before #6249. This removes this empty group from the generated NFA because it's completely unnecessary.

Drive-by change: Replace the first swap() call by explicit assignments, and qualify the other swap() call because the nodes are STL-internal types, so ADL is not needed here.

[Copilot]

Pull request overview

Note

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Removes unnecessary empty-group nodes from the generated NFA for ? quantifiers (including lazy ??), and adds a regression test to lock in the intended behavior.

Changes:

• Simplify NFA construction for ? by linking the empty alternative directly to _End instead of emitting an empty group.
• Adjust the non-greedy (??) pointer rewiring logic, using explicit assignments and _STD swap.
• Add GH-6267 tests for a? vs a?? match/search behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
tests/std/tests/VSO_0000000_regex_use/test.cpp	Adds GH-6267 regression coverage for greedy vs lazy optional quantifiers.
stl/inc/regex	Removes empty-group nodes for ? and tweaks non-greedy rewiring logic accordingly.

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[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo. Please notify me if any further changes are pushed, otherwise no action is required.

[StephanTLavavej]

Thanks for continuing to find ways to improve <regex>! 💚 😻 💝