[swift/main] Optimize search for start-anchored regexes #683
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When a regex is anchored to the start of a subject, there's no need to search throughout a string for the pattern when searching for the first match: a prefix match is sufficient. This adds a regex compilation-time check about whether a match can only be found at the start of a subject, and then uses that to choose whether to defer to `prefixMatch` from within `firstMatch`.
natecook1000 changed the title 
| @swift-ci Please test |
milseman approved these changes Jul 19, 2023
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When a regex is anchored to the start of a subject, there's no need to search throughout a string for the pattern when searching for the first match: a prefix match is sufficient.
This adds a regex compilation-time check about whether a match can only be found at the start of a subject, and then uses that to choose whether to defer to
prefixMatchfrom withinfirstMatch.(This is a cherry-pick of #682.)