Some more Regex cleanup and optimization (mostly source generator) #62426
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Tagging subscribers to this area: @dotnet/area-system-text-regularexpressions Issue Detailscc: @joperezr
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The expression "(?!)" (an empty negative lookahead) is sometimes used in expressions at a point where we want to match nothing and thus fail a match, often in combination with a conditional operation. This negative lookahead wrapped around an empty can be reduced to simply a Nothing, which is cheaper to process.
We currently output goto-based code, avoiding an actual if/else construct in case the child branches need to backtrack (and with the forced scoping for the if/else blocks, backtracking would result in compilation failures due to trying to jump to labels defined inside those scopes). But if we know in advance that no such backtracking will happen, we can output the nicer if/else code.
We were iterating through each individual character in our own open-coded loop, but for case-sensitive, we can just delegate to SequenceEqual.
Rather than wrapping a node in a new positive lookahead assertion node, just call the logic to emit the node as if it were a positive lookahead.
Ever so slightly cheaper
stephentoub
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joperezr
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Dec 6, 2021
src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.Emitter.cs
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joperezr
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Dec 6, 2021
danmoseley
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Dec 6, 2021
...libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/RegexCompiler.cs
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cc: @joperezr