Use the Regex source generator in the AlphaRouteConstraint.

src/Http/Routing/perf/Microbenchmarks/LinkGeneration/SingleRouteWithConstraintsBenchmark.cs

@@ -1,4 +1,4 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using BenchmarkDotNet.Attributes;
@@ -17,7 +17,7 @@ public class SingleRouteWithConstraintsBenchmark : EndpointRoutingBenchmarkBase
     [GlobalSetup]
     public void Setup()
     {
-        var template = "Customers/Details/{category}/{region}/{id:int}";
+        var template = "Customers/Details/{category:alpha}/{region:alpha}/{id:int}";
         var defaults = new { controller = "Customers", action = "Details" };
         var requiredValues = new { controller = "Customers", action = "Details" };
 

src/Http/Routing/src/Constraints/AlphaRouteConstraint.cs

@@ -1,17 +1,22 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Text.RegularExpressions;
+
 namespace Microsoft.AspNetCore.Routing.Constraints;
 
 /// <summary>
 /// Constrains a route parameter to contain only lowercase or uppercase letters A through Z in the English alphabet.
 /// </summary>
-public class AlphaRouteConstraint : RegexRouteConstraint
+public partial class AlphaRouteConstraint : RegexRouteConstraint
 {
     /// <summary>
     /// Initializes a new instance of the <see cref="AlphaRouteConstraint" /> class.
     /// </summary>
-    public AlphaRouteConstraint() : base(@"^[a-z]*$")
+    public AlphaRouteConstraint() : base(GetAlphaRouteRegex())
     {
     }
+
+    [GeneratedRegex(@"^[A-Za-z]*$", RegexOptions.CultureInvariant)]
+    private static partial Regex GetAlphaRouteRegex();
 }

src/Http/Routing/src/Constraints/RegexRouteConstraint.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.Text.RegularExpressions;
 using Microsoft.AspNetCore.Http;
@@ -33,7 +34,7 @@ public RegexRouteConstraint(Regex regex)
     /// Constructor for a <see cref="RegexRouteConstraint"/> given a <paramref name="regexPattern"/>.
     /// </summary>
     /// <param name="regexPattern">A string containing the regex pattern.</param>
-    public RegexRouteConstraint(string regexPattern)
+    public RegexRouteConstraint([StringSyntax(StringSyntaxAttribute.Regex)] string regexPattern)
     {
         if (regexPattern == null)
         {