lib/regexutil: vendor syntax standard lib in Go 1.23 as a workaround

[func25]

Describe Your Changes

Fix #6911

While waiting for the changes to be updated in Go's mainstream, I vendored the syntax package in Go 1.23, changed the call to the local syntax package, and the only modification to the vendored syntax package is the new calcFlagsI() function.

• Old version

case OpCharClass:
	// If literal is fold-sensitive, return 0, flagI - (?i) has been compiled out.
	// If literal is not fold-sensitive, return 0, 0.
	for i := 0; i < len(re.Rune); i += 2 {
		lo := max(minFold, re.Rune[i])
		hi := min(maxFold, re.Rune[i+1])
		for r := lo; r <= hi; r++ {
			for f := unicode.SimpleFold(r); f != r; f = unicode.SimpleFold(f) {
				if !(lo <= f && f <= hi) && !inCharClass(f, re.Rune) {
					return 0, flagI
				}
			}
		}
	}
	return 0, 0

New version:

func calcFlagsI(re *Regexp) (must, cant printFlags) { ... }

[valyala]

The approach with vendoring of regexp/syntax package with the bugfix looks interesting as a temporary workaround for the issue in this package!

[valyala]

Please refer the created issue at Go repository here.

[func25]

Go issue: #69303
Go CL: #611300

[hagen1778]

Let's put this PR on-hold. The change seems too intrusive, while the effect of the mentioned bug seems pretty low. Probably, we should wait for this to be fixed on Go side and the use newer Go version instead.