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`<regex>`: Speed up searches for regexes that start with assertions by muellerj2 Β· Pull Request #5576 Β· microsoft/STL Β· GitHub
StephanTLavavej
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Must go faster
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meow is a substring of homeowner
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<regex>
: Speed up searches for regexes that start with assertions
#5576
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Merged
StephanTLavavej
merged 2 commits into
microsoft:main
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muellerj2:regex-speedup-assertion-searches
Jun 14, 2025
Merged
<regex>
: Speed up searches for regexes that start with assertions
#5576
StephanTLavavej
merged 2 commits into
microsoft:main
from
muellerj2:regex-speedup-assertion-searches
Jun 14, 2025
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StephanTLavavej
approved these changes
Jun 10, 2025
I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed. |
StephanTLavavej
approved these changes
Jun 14, 2025
I resolved a trivial adjacent-add conflict with #5535 in |
Must go faster! π π¦ π» |
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Towards #5468. This extends the skip heuristic to the remaining unhandled assertions: word boundaries and lookahead assertions.
_Matcher2::_Skip()
than in_Matcher2::_Is_wbound()
, because we don't have to do any special handling for the start or end of the input string:_Skip()
is never called for the start (or as the comment at the top says,--_First_arg
is valid) and it cannot skip beyond the end of the input string (_Last
) anyway. So we only have to handle the middle case and have to keep looking for the first position where the word character property changes (\b
) or where it does not change (\B
)._Skip()
is just a heuristic, so it's fine if we don't exclude some non-matches. There is only one thing we must not do here: Skip something that could match.<regex>
: Nonlinear slowdown with increasing string lengthΒ #5452, this does not suffer from the same problem because we only move forward in the checks: If evaluating the start of the assertion body or the regex after the assertion body tells us that we should skip until position x, then we search next for the first possible position the other part of the regex matches starting from x. No position is evaluated more than once for the assertion and once for the remainder of the regex after the assertion, which implies a running time linear in the input._N_endif
nodes. (Nothing relevant happens at such nodes. They don't imply anything about the input string. Their only feature is that all the branches of an_N_if
node join here again, but the problematic case is when branching happens, not when branches end.)<regex>
: Avoid generating unnecessary single-branch_N_if
nodesΒ #5539, this doesn't actually make a difference if the current matcher and parser are combined. It can improve performance, though, when the current matcher evaluates an NFA generated by an old parser.Tests
I added some tests for lookahead assertions. The tests use
regex_replace()
because it performs several searches into a single test call.There already was test coverage for skipping of word boundaries in
VSO_0000000_regex_use
'stest_DDB_153116_replacements()
function. But the test coverage had a gap: It didn't check for correct behavior if two (or more) non-word characters are next to each other. I closed this gap by adding some spaces to the existing tests.Benchmark