Replace a few Regex static method calls

dotnet/src/SemanticKernel/Planning/Plan.cs

@@ -367,7 +367,7 @@ public ISKFunction SetAIConfiguration(CompleteRequestSettings settings)
     internal string ExpandFromVariables(ContextVariables variables, string input)
     {
         var result = input;
-        var matches = Regex.Matches(input, @"\$(?<var>\w+)");
+        var matches = s_variablesRegex.Matches(input);
         var orderedMatches = matches.Cast<Match>().Select(m => m.Groups["var"].Value).OrderByDescending(m => m.Length);
 
         foreach (var varName in orderedMatches)
@@ -489,4 +489,6 @@ private void SetFunction(ISKFunction function)
     private ISKFunction? Function { get; set; } = null;
 
     private readonly List<Plan> _steps = new();
+
+    private static readonly Regex s_variablesRegex = new(@"\$(?<var>\w+)");
 }

dotnet/src/SemanticKernel/TemplateEngine/Blocks/FunctionIdBlock.cs

@@ -38,7 +38,7 @@ public FunctionIdBlock(string? text, ILogger? log = null)
 
     public override bool IsValid(out string errorMsg)
     {
-        if (!Regex.IsMatch(this.Content, "^[a-zA-Z0-9_.]*$"))
+        if (!s_validContentRegex.IsMatch(this.Content))
         {
             errorMsg = "The function identifier is empty";
             return false;
@@ -66,4 +66,6 @@ private static bool HasMoreThanOneDot(string? value)
         int count = 0;
         return value.Any(t => t == '.' && ++count > 1);
     }
+
+    private static readonly Regex s_validContentRegex = new("^[a-zA-Z0-9_.]*$");
 }

dotnet/src/SemanticKernel/TemplateEngine/Blocks/VarBlock.cs

@@ -50,7 +50,7 @@ public override bool IsValid(out string errorMsg)
             return false;
         }
 
-        if (!Regex.IsMatch(this.Name, "^[a-zA-Z0-9_]*$"))
+        if (!s_validNameRegex.IsMatch(this.Name))
         {
             errorMsg = $"The variable name '{this.Name}' contains invalid characters. " +
                        "Only alphanumeric chars and underscore are allowed.";
@@ -78,4 +78,6 @@ public string Render(ContextVariables? variables)
 
         return exists ? value : string.Empty;
     }
+
+    private static readonly Regex s_validNameRegex = new("^[a-zA-Z0-9_]*$");
 }

dotnet/src/Skills/Skills.OpenAPI/Extensions/RestApiOperationExtensions.cs

@@ -57,11 +57,12 @@ public static IReadOnlyList<RestApiOperationParameter> GetParameters(this RestAp
         //Create a property alternative name without special symbols that are not supported by SK template language.
         foreach (var parameter in parameters)
         {
-            parameter.AlternativeName = Regex.Replace(parameter.Name, @"[^0-9A-Za-z_]+", "_");
+            parameter.AlternativeName = s_invalidSymbolsRegex.Replace(parameter.Name, "_");
         }
 
         return parameters;
     }
 
     private const string MediaTypeTextPlain = "text/plain";
+    private static readonly Regex s_invalidSymbolsRegex = new(@"[^0-9A-Za-z_]+");
 }