[5.9] Optimize search for start-anchored regexes #684
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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`.
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natecook1000
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The order of this property in MEProgram seems to determine whether or not it persists from the time it's stored to when it's accessed in RegexDSLTests. Clearly something else is going on here, but this works around the issue for now.
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| /// - `nil`: This node is inconclusive about where it can match. | ||
| /// | ||
| /// In particular, non-required groups and option-setting groups are | ||
| /// inconclusive about where they can match. |
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How is nil actually different from false? What can the caller do with a nil result that they cannot do with a false result?
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Returning nil means that the node doesn't determine the result either way. For example, in /(?i)^foo/, the (?i) node is encountered as the first child of a concatenation, and doesn't affect whether or not the pattern can only match at the start. Returning nil means that the search for a definitive answer can continue to the next sibling.
Note that callers don't have access to this method; the one they call just returns a Bool.
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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.)