[hippi345]

Summary

Matching (a){0}b (and (a){0,0}b) against "b" reported the capture as an empty match ("") instead of nil. A group quantified {0} never participates, so its capture must stay unset, consistent with (a)?b, (a)*b, and (a){0,1}b.

The {0} / {0,0} compiler path dropped the quantified body but still emitted begin/end capture around the skip, which stored an empty range at the current position. This change still drops the never-run body and still allocates capture registers (so later groups keep the correct numbering), but it no longer begins or ends those captures.

Fixes #902

Changes

• In emitQuantification, skip a {0} / {0,0} operand with preservingCaptures: false and allocatingCaptures: true.
• skipNode allocates nested capture registers when requested, without recording an empty match.
• Added flatCaptureTest coverage for (a){0}b, (a){0,0}b, nested ((a)){0}(b), and a {0,1} regression check.

Testing

Fail-before (unmodified compiler, swift test --filter testMatchCaptureBehavior):

Test Case 'RegexTests.testMatchCaptureBehavior' started at 2026-10-17 02:44:24.745
/workspace/Tests/RegexTests/MatchTests.swift:2085: error: RegexTests.testMatchCaptureBehavior : failed - MatchError(message: "Capture mismatch:\n  [nil]\n  [Optional(\"\")]")
/workspace/Tests/RegexTests/MatchTests.swift:2085: error: RegexTests.testMatchCaptureBehavior : failed - MatchError(message: "Capture mismatch:\n  [nil]\n  [Optional(\"\")]")
/workspace/Tests/RegexTests/MatchTests.swift:2089: error: RegexTests.testMatchCaptureBehavior : failed - MatchError(message: "Capture mismatch:\n  [nil]\n  [Optional(\"\")]")
/workspace/Tests/RegexTests/MatchTests.swift:2089: error: RegexTests.testMatchCaptureBehavior : failed - MatchError(message: "Capture mismatch:\n  [nil]\n  [Optional(\"\")]")
/workspace/Tests/RegexTests/MatchTests.swift:2095: error: RegexTests.testMatchCaptureBehavior : failed - MatchError(message: "Capture mismatch:\n  [nil, nil, Optional(\"b\")]\n  [Optional(\"\"), Optional(\"\"), Optional(\"b\")]")
Test Case 'RegexTests.testMatchCaptureBehavior' failed (0.053 seconds)
Test Suite 'RegexTests' failed at 2026-10-17 02:44:24.799
	 Executed 1 test, with 5 failures (0 unexpected) in 0.053 (0.053) seconds

Pass-after (swift test --filter testMatchCaptureBehavior, then RegexTests target):

Test Case 'RegexTests.testMatchCaptureBehavior' started at 2026-10-17 02:44:46.473
Test Case 'RegexTests.testMatchCaptureBehavior' passed (0.05 seconds)
Test Suite 'RegexTests' passed at 2026-10-17 02:44:46.523
	 Executed 1 test, with 0 failures (0 unexpected) in 0.05 (0.05) seconds

Test Suite 'RegexTests' passed at 2026-10-17 02:45:03.397
	 Executed 76 tests, with 0 failures (0 unexpected) in 2.657 (2.657) seconds
Test Suite 'Selected tests' passed at 2026-10-17 02:45:03.661
	 Executed 130 tests, with 0 failures (0 unexpected) in 3.759 (3.759) seconds

This change was prepared with AI assistance and reviewed by the author.