perf: pre-compile regex in RegexMatch for better performance

[Herrtian]

Summary

Ref #1690

RegexMatch() used regexp.MatchString() which recompiles the regex pattern on every single invocation. This is a performance bottleneck since RegexMatch is called by KeyMatch2, KeyMatch3, KeyMatch5, and can be used directly in policy matchers.

Change

Replaced regexp.MatchString() with the existing mustCompileOrGet() cache that is already used throughout the codebase for other KeyMatch functions. The compiled regex is cached in a thread-safe map and reused on subsequent calls.

// Before: recompiles regex on every call
func RegexMatch(key1 string, key2 string) bool {
    res, err := regexp.MatchString(key2, key1)
    if err != nil {
        panic(err)
    }
    return res
}

// After: uses cached compiled regex
func RegexMatch(key1 string, key2 string) bool {
    return mustCompileOrGet(key2).MatchString(key1)
}

Test plan

• go build ./... passes
• go test ./... — all tests pass
• go test ./util/... — all util tests pass including regex match tests

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