<regex>: Avoid generating group nodes for non-capturing groups

[muellerj3]

Towards #5962. Also helps a bit with #6347.

This completes the removal of _N_group and _N_end_group node generation from the parser (but have to continue handling such nodes in the matcher for ABI reasons). As a side effect, this also avoids generating any nodes for constructs like (?:)*.

This is achieved by marking the position of a node just before a non-capturing or capturing group and using this mark to generate the repeat nodes around such a group if the group is followed by a quantifier. We store this mark in _Group_start. If _Group_start is null, we know that the quantifier doesn't apply to a group, so the repeat nodes have to surround the node generated last only.

_Group_start is reset after handling the quantifier, so it can't accidentally affect the parsing of following alternatives. We also have to assign the mark to _Group_start after the group has been fully parsed, otherwise the mark might be accidentally used in or reset by the recursive _Alternative() call.

The code that handles a quantifier following a non-group already handles the generation of _N_str nodes correctly. We have to handle this for a repeated group now as well: _Builder3::_Chars might not be empty for a non-capturing group because we do not generate an _N_end_group node anymore that triggers the generation of the _N_str node. This means that _Emit_str_node() must be called first in _Add_group_rep().

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[Copilot]

Pull request overview

Removes parser-generated no-op nodes for non-capturing regex groups while preserving ABI handling.

Changes:

• Tracks group boundaries to generate repetition nodes correctly.
• Splits grouped and non-grouped repetition handling.
• Adds regression coverage for grouped quantifiers and captures.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
stl/inc/regex	Removes non-capturing group nodes and revises repetition generation.
tests/std/tests/VSO_0000000_regex_use/test.cpp	Adds grouped-quantifier regression tests.

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[muellerj3]

This is a good catch of a very subtle problem (even if the mishandled pattern seems very unlikely to appear in practice).

But I think this only affects positive assertions, because capture groups in negative assertions are never matched after the assertion has been processed.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.