Reduce allocations in LicenseExpressionTokenizer.HasValidCharacters by caching Regex instance (#7237)

Author: nareshjo

src/NuGet.Core/NuGet.Packaging/Licenses/LicenseExpressionTokenizer.cs

@@ -10,6 +10,7 @@ namespace NuGet.Packaging.Licenses
 {
     internal class LicenseExpressionTokenizer
     {
+        private static readonly Regex ValidCharactersRegex = new Regex("^[a-zA-Z0-9\\.\\-\\s\\+\\(\\)]+$", RegexOptions.CultureInvariant);
         private readonly string _value;
 
         /// <summary>
@@ -34,8 +35,7 @@ internal LicenseExpressionTokenizer(string value)
         /// <returns>Whether the value has valid characters.</returns>
         internal bool HasValidCharacters()
         {
-            var regex = new Regex("^[a-zA-Z0-9\\.\\-\\s\\+\\(\\)]+$", RegexOptions.CultureInvariant);
-            return regex.IsMatch(_value);
+            return ValidCharactersRegex.IsMatch(_value);
         }
 
         /// <summary>