Use static Regex.IsMatch in ValueMatchFilter to take advantage of caching

[stevenaw]

The static Regex functions internally cache parsed regex instances based on the provided pattern on .NET 5+. ValueMatchFilter can be used to filter tests based on a regex and will apply the same regex to all potential candidates to see if it matches. This means that on a large test suite there could be measurable savings by using an overload which caches the regex.

Something like this:
https://github.com/nunit/nunit/blob/master/src%2FNUnitFramework%2Fframework%2FInternal%2FFilters%2FValueMatchFilter.cs#L43

-return input != null && new Regex(ExpectedValue).IsMatch(input);
+return input != null && Regex.IsMatch(input, ExpectedValue);

[stevenaw]

Alternately, we could consider implementing our own caching mechanism so that all runtimes can benefit

[manfred-brands]

I think it is much simpler than that. ExpectedValue is a constant initialized in the constructor.
Instead of using it as a string, we an create a (compiled) RegEx instance here and called IsMatch on that instance.
I find the IsRegEx property a bit weird. The pattern can't change type halfway through. Looking at its usages it is mainly used as an init property. In my opinion it should be changed to a constructor parameter.

This matches the pattern that is already used in StackFilter and RegexConstaint.

I will create a PR.

[stevenaw]

Thanks @manfred-brands ! That was exactly what I had been thinking too, right down to setting IsRegEx as a ctor param. Thanks for offering to take this on 😁