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Reset the accumulated minimum when collapsing an optional loop so repeated empty alternatives retain their zero minimum. Add reduction and multi-engine functional coverage. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com> Copilot-Session: d76b2ba7-1921-4e29-b0b3-77ad73e9c582
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This was referenced Oct 7, 2026
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.NET 9: https://mihubot.xyz/regex?pattern=%28%3F%3Aa%2B%7C%29%7B2%7D&version=9.0
.NET 10: https://mihubot.xyz/regex?pattern=%28%3F%3Aa%2B%7C%29%7B2%7D&version=10.0
Repeated empty alternatives such as
(?:a+|){2}incorrectly reject"a". When collapsing nested loops,ReduceLoopsskips updating its accumulated minimum for a zero-minimum child. A subsequent merge then incorrectly reduces the pattern toa{2,}instead ofa*.Reset the accumulated minimum to zero when crossing an optional loop. Maximum counts, overflow handling, and the guards determining whether loops can safely be combined remain unchanged. The correction applies to interpreted, compiled, source-generated, and nonbacktracking regexes through their shared optimizer.
The empty-alternation optimization in #118087 exposed this preexisting accumulator defect.
Resolves #135329
Note
This change and description were prepared with GitHub Copilot.