<regex>: Fix quadratic complexity in regex_search when regex starts with ? quantifier or several alternatives
#5457
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…ts with `?` quantifier or several alternatives
AlexGuteniev
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May 1, 2025
StephanTLavavej
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performance
StephanTLavavej
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May 3, 2025
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Locally verified too. Given the very clear comment, and the difficulty of verifying this in automated testing without burning a bunch of CPU time, I'm okay with not having test coverage for this. |
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I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed. |
StephanTLavavej
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May 9, 2025
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O(Thanks2) 😹 🚀 🎉 |
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Fixes #5452. This is the minimal fix in
_Matcher::_Skipthat I mentioned in the issue: It avoids worst-case running time quadratic in the size of the searched string by giving up on_N_ifnodes with two or more branches.Unfortunately, we can't just give up whenever an
_N_ifis encountered because the parser currently likes to generate such nodes with a single branch. In particular, there is one such node at the beginning of each generated NFA. This means that giving up on such nodes would render_Skippointless for all regular expressions.I confirmed locally that this change is sufficient to fix the quadratic running time that #5452's test case exposes (though the quantifiers still almost double the running time.)