<regex>: Use std::search() in skip heuristic by muellerj2 · Pull Request #5586 · microsoft/STL (original) (raw)

Towards #5468. This replaces the weird usage of _Cmp_chrange() by a straightforward call to std::search() in _Matcher2::_Skip().

I fix a copy-paste mistake in the comment describing _Cmp_icase_translateleft as well.

I also made an attempt to replace _Cmp_chrange()'s implementation by a straightforward call to std::mismatch(), but that seems to be a pessimization in practice of about 10 % (probably because the strings tend to be quite short).

There will still be one follow-up to make an obvious improvement to the skip heuristic for regex and wregex in collate mode. But otherwise, I think this is basically it for simple improvements to the skip heuristic. There are still a few opportunities that could lead to some improvement -- handling several branches, avoiding to walk the NFA for each _Skip() call, or avoiding to compare the NFA nodes matched by _Skip() in _Match_pat() again -- but they are not straightforward to implement.

Benchmark

Note that this means that all improvements since #5509 have sped up searching for the regex (bibe)+ by a factor of about 450 and for (?:bibe)+ by a factor of about 1000 in this benchmark.