[Analyzer]: Regex.Matches(...).Count to Regex.Count(...)

[stephentoub]

We should have an analyzer that looks for:

Regex.Matches(...).Count // or Count()

and replaces it with:

Regex.Count(...);

when the target environment has a Count overload with parameters that exactly match those of the used Matches overload. This applies to both the static and instance methods. Computing the Count directly is much cheaper than using Matches as it doesn't need to allocate all the Match objects, doesn't need to allocate the collection, and doesn't need to compute information like capture groups.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-text-regularexpressions
See info in area-owners.md if you want to be subscribed.