Exclude regex and alpha constraints when SlimBuilder is used.

src/DefaultBuilder/src/WebApplicationBuilder.cs

@@ -134,7 +134,7 @@ internal WebApplicationBuilder(WebApplicationOptions options, bool slim, Action<
         bootstrapHostBuilder.ConfigureSlimWebHost(
             webHostBuilder =>
             {
-                AspNetCore.WebHost.UseKestrel(webHostBuilder);
+                AspNetCore.WebHost.ConfigureWebDefaultsCore(webHostBuilder);
 
                 webHostBuilder.Configure(ConfigureEmptyApplication);
 

src/DefaultBuilder/src/WebHost.cs

@@ -223,14 +223,17 @@ internal static void ConfigureWebDefaults(IWebHostBuilder builder)
             }
         });
 
-        UseKestrel(builder);
+        ConfigureWebDefaultsCore(builder, services =>
+        {
+            services.AddRouting();
+        });
 
         builder
             .UseIIS()
             .UseIISIntegration();
     }
 
-    internal static void UseKestrel(IWebHostBuilder builder)
+    internal static void ConfigureWebDefaultsCore(IWebHostBuilder builder, Action<IServiceCollection>? configureRouting = null)
     {
         builder.UseKestrel((builderContext, options) =>
         {
@@ -257,7 +260,17 @@ internal static void UseKestrel(IWebHostBuilder builder)
             services.AddTransient<IStartupFilter, ForwardedHeadersStartupFilter>();
             services.AddTransient<IConfigureOptions<ForwardedHeadersOptions>, ForwardedHeadersOptionsSetup>();
 
-            services.AddRouting();
+            // Provide a way for the default host builder to configure routing. This probably means calling AddRouting.
+            // A lambda is used here because we don't want to reference AddRouting directly because of trimming.
+            // This avoids the overhead of calling AddRoutingCore multiple times on app startup.
+            if (configureRouting == null)
+            {
+                services.AddRoutingCore();
+            }
+            else
+            {
+                configureRouting(services);
+            }
         });
     }
 

src/DefaultBuilder/test/Microsoft.AspNetCore.Tests/WebApplicationTests.cs

@@ -20,6 +20,7 @@
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Http.Features;
 using Microsoft.AspNetCore.Routing;
+using Microsoft.AspNetCore.Routing.Constraints;
 using Microsoft.AspNetCore.TestHost;
 using Microsoft.AspNetCore.Testing;
 using Microsoft.AspNetCore.Tests;
@@ -2267,6 +2268,141 @@ public async Task SupportsDisablingMiddlewareAutoRegistration()
         Assert.True(app.Properties.ContainsKey("__AuthorizationMiddlewareSet"));
     }
 
+    [Fact]
+    public async Task UsingCreateBuilderResultsInRegexConstraintBeingPresent()
+    {
+        var builder = WebApplication.CreateBuilder();
+        builder.WebHost.UseTestServer();
+
+        var app = builder.Build();
+
+        var chosenRoute = string.Empty;
+
+        app.Use((context, next) =>
+        {
+            chosenRoute = context.GetEndpoint()?.DisplayName;
+            return next(context);
+        });
+
+        app.MapGet("/products/{productId:regex(^[a-z]{{4}}\\d{{4}}$)}", (string productId) => productId).WithDisplayName("RegexRoute");
+
+        await app.StartAsync();
+
+        var client = app.GetTestClient();
+
+        _ = await client.GetAsync("https://localhost/products/abcd1234");
+        Assert.Equal("RegexRoute", chosenRoute);
+    }
+
+    [Fact]
+    public async Task UsingCreateSlimBuilderResultsInAlphaConstraintStillWorking()
+    {
+        var builder = WebApplication.CreateSlimBuilder();
+        builder.WebHost.UseTestServer();
+
+        var app = builder.Build();
+
+        var chosenRoute = string.Empty;
+
+        app.Use((context, next) =>
+        {
+            chosenRoute = context.GetEndpoint()?.DisplayName;
+            return next(context);
+        });
+
+        app.MapGet("/products/{productId:alpha:minlength(4):maxlength(4)}", (string productId) => productId).WithDisplayName("AlphaRoute");
+
+        await app.StartAsync();
+
+        var client = app.GetTestClient();
+
+        _ = await client.GetAsync("https://localhost/products/abcd");
+        Assert.Equal("AlphaRoute", chosenRoute);
+    }
+
+    [Fact]
+    public async Task UsingCreateSlimBuilderResultsInErrorWhenTryingToUseRegexConstraint()
+    {
+        var builder = WebApplication.CreateSlimBuilder();
+        builder.WebHost.UseTestServer();
+
+        var app = builder.Build();
+
+        app.MapGet("/products/{productId:regex(^[a-z]{{4}}\\d{{4}}$)}", (string productId) => productId).WithDisplayName("AlphaRoute");
+
+        await app.StartAsync();
+
+        var client = app.GetTestClient();
+
+        var ex = await Record.ExceptionAsync(async () =>
+        {
+            _ = await client.GetAsync("https://localhost/products/abcd1234");
+        });
+
+        Assert.IsType<RouteCreationException>(ex);
+        Assert.IsType<InvalidOperationException>(ex.InnerException.InnerException);
+        Assert.Equal(
+            "A route parameter uses the regex constraint, which isn't registered. If this application was configured using CreateSlimBuilder(...) or AddRoutingCore(...) then this constraint is not registered by default. To use the regex constraint, configure route options at app startup: services.Configure<RouteOptions>(options => options.SetParameterPolicy<RegexInlineRouteConstraint>(\"regex\"));",
+            ex.InnerException.InnerException.Message);
+    }
+
+    [Fact]
+    public async Task UsingCreateSlimBuilderWorksIfRegexConstraintAddedViaAddRouting()
+    {
+        var builder = WebApplication.CreateSlimBuilder();
+        builder.Services.AddRouting();
+        builder.WebHost.UseTestServer();
+
+        var app = builder.Build();
+
+        var chosenRoute = string.Empty;
+
+        app.Use((context, next) =>
+        {
+            chosenRoute = context.GetEndpoint()?.DisplayName;
+            return next(context);
+        });
+
+        app.MapGet("/products/{productId:regex(^[a-z]{{4}}\\d{{4}}$)}", (string productId) => productId).WithDisplayName("RegexRoute");
+
+        await app.StartAsync();
+
+        var client = app.GetTestClient();
+
+        _ = await client.GetAsync("https://localhost/products/abcd1234");
+        Assert.Equal("RegexRoute", chosenRoute);
+    }
+
+    [Fact]
+    public async Task UsingCreateSlimBuilderWorksIfRegexConstraintAddedViaAddRoutingCoreWithActionDelegate()
+    {
+        var builder = WebApplication.CreateSlimBuilder();
+        builder.Services.AddRoutingCore().Configure<RouteOptions>(options =>
+        {
+            options.SetParameterPolicy<RegexInlineRouteConstraint>("regex");
+        });
+        builder.WebHost.UseTestServer();
+
+        var app = builder.Build();
+
+        var chosenRoute = string.Empty;
+
+        app.Use((context, next) =>
+        {
+            chosenRoute = context.GetEndpoint()?.DisplayName;
+            return next(context);
+        });
+
+        app.MapGet("/products/{productId:regex(^[a-z]{{4}}\\d{{4}}$)}", (string productId) => productId).WithDisplayName("RegexRoute");
+
+        await app.StartAsync();
+
+        var client = app.GetTestClient();
+
+        _ = await client.GetAsync("https://localhost/products/abcd1234");
+        Assert.Equal("RegexRoute", chosenRoute);
+    }
+
     private class UberHandler : AuthenticationHandler<AuthenticationSchemeOptions>
     {
         public UberHandler(IOptionsMonitor<AuthenticationSchemeOptions> options, ILoggerFactory logger, UrlEncoder encoder, ISystemClock clock) : base(options, logger, encoder, clock) { }

src/DefaultBuilder/test/Microsoft.AspNetCore.Tests/WebHostTests.cs

@@ -7,6 +7,7 @@
 using Microsoft.AspNetCore.HostFiltering;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Routing;
+using Microsoft.AspNetCore.Routing.Constraints;
 using Microsoft.AspNetCore.TestHost;
 using Microsoft.AspNetCore.Testing;
 using Microsoft.Extensions.Configuration;
@@ -110,6 +111,19 @@ public void CreateDefaultBuilder_RegistersEventSourceLogger()
             args.Payload.OfType<string>().Any(p => p.Contains("Request starting")));
     }
 
+    [Fact]
+    public void WebHost_CreateDefaultBuilder_ConfiguresRegexInlineRouteConstraint_ByDefault()
+    {
+        var host = WebHost.CreateDefaultBuilder()
+            .Configure(_ => { })
+            .Build();
+
+        var routeOptions = host.Services.GetService<IOptions<RouteOptions>>();
+
+        Assert.True(routeOptions.Value.ConstraintMap.ContainsKey("regex"));
+        Assert.Equal(typeof(RegexInlineRouteConstraint), routeOptions.Value.ConstraintMap["regex"]);
+    }
+
     private class TestEventListener : EventListener
     {
         private volatile bool _disposed;

src/Http/Routing/src/Constraints/RegexErrorStubRouteConstraint.cs

@@ -0,0 +1,20 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Microsoft.AspNetCore.Http;
+
+namespace Microsoft.AspNetCore.Routing.Constraints;
+
+internal sealed class RegexErrorStubRouteConstraint : IRouteConstraint
+{
+    public RegexErrorStubRouteConstraint(string _)
+    {
+        throw new InvalidOperationException(Resources.RegexRouteContraint_NotConfigured);
+    }
+
+    bool IRouteConstraint.Match(HttpContext? httpContext, IRouter? route, string routeKey, RouteValueDictionary values, RouteDirection routeDirection)
+    {
+        // Should never get called, but is same as throw in constructor in case constructor is changed.
+        throw new InvalidOperationException(Resources.RegexRouteContraint_NotConfigured);
+    }
+}

src/Http/Routing/src/DependencyInjection/RoutingServiceCollectionExtensions.cs

@@ -26,6 +26,20 @@ public static class RoutingServiceCollectionExtensions
     /// <param name="services">The <see cref="IServiceCollection"/> to add the services to.</param>
     /// <returns>The <see cref="IServiceCollection"/> so that additional calls can be chained.</returns>
     public static IServiceCollection AddRouting(this IServiceCollection services)
+    {
+        services.AddRoutingCore();
+        services.TryAddEnumerable(ServiceDescriptor.Singleton<IConfigureOptions<RouteOptions>, RegexInlineRouteConstraintSetup>());
+        return services;
+    }
+
+    /// <summary>
+    /// Adds services required for routing requests. This is similar to
+    /// <see cref="AddRouting(IServiceCollection)" /> except that it
+    /// excludes certain options that can be opted in separately, if needed.
+    /// </summary>
+    /// <param name="services">The <see cref="IServiceCollection"/> to add the services to.</param>
+    /// <returns>The <see cref="IServiceCollection"/> so that additional calls can be chained.</returns>
+    public static IServiceCollection AddRoutingCore(this IServiceCollection services)
     {
         ArgumentNullException.ThrowIfNull(services);
 

src/Http/Routing/src/Microsoft.AspNetCore.Routing.csproj

@@ -50,5 +50,6 @@
     <InternalsVisibleTo Include="Microsoft.AspNetCore.Routing.Tests" />
     <InternalsVisibleTo Include="Microsoft.AspNetCore.Mvc.ApiExplorer.Test" />
     <InternalsVisibleTo Include="DynamicProxyGenAssembly2" Key="$(MoqPublicKey)" />
+    <InternalsVisibleTo Include="Microsoft.AspNetCore.Mvc.Test" />
   </ItemGroup>
 </Project>

src/Http/Routing/src/PublicAPI.Unshipped.txt

@@ -1,3 +1,4 @@
 #nullable enable
 Microsoft.AspNetCore.Routing.RouteHandlerServices
 static Microsoft.AspNetCore.Routing.RouteHandlerServices.Map(Microsoft.AspNetCore.Routing.IEndpointRouteBuilder! endpoints, string! pattern, System.Delegate! handler, System.Collections.Generic.IEnumerable<string!>! httpMethods, System.Func<System.Reflection.MethodInfo!, Microsoft.AspNetCore.Http.RequestDelegateFactoryOptions?, Microsoft.AspNetCore.Http.RequestDelegateMetadataResult!>! populateMetadata, System.Func<System.Delegate!, Microsoft.AspNetCore.Http.RequestDelegateFactoryOptions!, Microsoft.AspNetCore.Http.RequestDelegateMetadataResult?, Microsoft.AspNetCore.Http.RequestDelegateResult!>! createRequestDelegate) -> Microsoft.AspNetCore.Builder.RouteHandlerBuilder!
+static Microsoft.Extensions.DependencyInjection.RoutingServiceCollectionExtensions.AddRoutingCore(this Microsoft.Extensions.DependencyInjection.IServiceCollection! services) -> Microsoft.Extensions.DependencyInjection.IServiceCollection!

src/Http/Routing/src/RegexInlineRouteConstraintSetup.cs

@@ -0,0 +1,24 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Microsoft.AspNetCore.Routing.Constraints;
+using Microsoft.AspNetCore.Routing;
+using Microsoft.Extensions.Options;
+
+namespace Microsoft.Extensions.DependencyInjection;
+
+internal sealed class RegexInlineRouteConstraintSetup : IConfigureOptions<RouteOptions>
+{
+    public void Configure(RouteOptions options)
+    {
+        var existingRegexConstraintType = options.TrimmerSafeConstraintMap["regex"];
+
+        // Don't override regex constraint if it has already been overridden
+        // this behavior here is just to add it back in if someone calls AddRouting(...)
+        // after setting up routing with AddRoutingCore(...).
+        if (existingRegexConstraintType == typeof(RegexErrorStubRouteConstraint))
+        {
+            options.SetParameterPolicy<RegexInlineRouteConstraint>("regex");
+        }
+    }
+}

src/Http/Routing/src/Resources.resx

@@ -249,4 +249,7 @@
   <data name="RouteEndpointDataSource_RequestDelegateCannotBeCalledBeforeBuild" xml:space="preserve">
     <value>This RequestDelegate cannot be called before the final endpoint is built.</value>
   </data>
+  <data name="RegexRouteContraint_NotConfigured" xml:space="preserve">
+    <value>A route parameter uses the regex constraint, which isn't registered. If this application was configured using CreateSlimBuilder(...) or AddRoutingCore(...) then this constraint is not registered by default. To use the regex constraint, configure route options at app startup: services.Configure&lt;RouteOptions&gt;(options =&gt; options.SetParameterPolicy&lt;RegexInlineRouteConstraint&gt;("regex"));</value>
+  </data>
 </root>

src/Http/Routing/src/RouteOptions.cs

@@ -113,9 +113,10 @@ private static IDictionary<string, Type> GetDefaultConstraintMap()
         AddConstraint<MaxRouteConstraint>(defaults, "max");
         AddConstraint<RangeRouteConstraint>(defaults, "range");
 
-        // Regex-based constraints
+        // The alpha constraint uses a compiled regex which has a minimal size cost.
         AddConstraint<AlphaRouteConstraint>(defaults, "alpha");
-        AddConstraint<RegexInlineRouteConstraint>(defaults, "regex");
+
+        AddConstraint<RegexErrorStubRouteConstraint>(defaults, "regex"); // Used to generate error message at runtime with helpful message.
 
         AddConstraint<RequiredRouteConstraint>(defaults, "required");
 

src/Http/Routing/test/UnitTests/DefaultInlineConstraintResolverTest.cs

@@ -16,6 +16,8 @@ public class DefaultInlineConstraintResolverTest
     public DefaultInlineConstraintResolverTest()
     {
         var routeOptions = new RouteOptions();
+        routeOptions.SetParameterPolicy<RegexInlineRouteConstraint>("regex");
+
         _constraintResolver = GetInlineConstraintResolver(routeOptions);
     }
 

src/Http/Routing/test/UnitTests/Matching/DfaMatcherBuilderTest.cs

@@ -3546,7 +3546,8 @@ private static DefaultParameterPolicyFactory CreateParameterPolicyFactory()
                 ConstraintMap =
                 {
                         ["slugify"] = typeof(SlugifyParameterTransformer),
-                        ["upper-case"] = typeof(UpperCaseParameterTransform)
+                        ["upper-case"] = typeof(UpperCaseParameterTransform),
+                        ["regex"] = typeof(RegexInlineRouteConstraint) // Regex not included by default since introduction of CreateSlimBuilder
                 }
             }),
             serviceCollection.BuildServiceProvider());

src/Http/Routing/test/UnitTests/Matching/RouteMatcherBuilder.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Routing.Constraints;
 using Microsoft.AspNetCore.Routing.Patterns;
 using Microsoft.AspNetCore.Routing.TestObjects;
 using Microsoft.Extensions.Options;
@@ -15,7 +16,9 @@ internal class RouteMatcherBuilder : MatcherBuilder
 
     public RouteMatcherBuilder()
     {
-        _constraintResolver = new DefaultInlineConstraintResolver(Options.Create(new RouteOptions()), new TestServiceProvider());
+        var routeOptions = new RouteOptions();
+        routeOptions.SetParameterPolicy<RegexInlineRouteConstraint>("regex");
+        _constraintResolver = new DefaultInlineConstraintResolver(Options.Create(routeOptions), new TestServiceProvider());
         _endpoints = new List<RouteEndpoint>();
     }
 

src/Http/Routing/test/UnitTests/Matching/TreeRouterMatcherBuilder.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Routing.Constraints;
 using Microsoft.AspNetCore.Routing.Template;
 using Microsoft.AspNetCore.Routing.TestObjects;
 using Microsoft.AspNetCore.Routing.Tree;
@@ -27,10 +28,13 @@ public override void AddEndpoint(RouteEndpoint endpoint)
 
     public override Matcher Build()
     {
+        var routeOptions = new RouteOptions();
+        routeOptions.SetParameterPolicy<RegexInlineRouteConstraint>("regex");
+
         var builder = new TreeRouteBuilder(
             NullLoggerFactory.Instance,
             new DefaultObjectPool<UriBuildingContext>(new UriBuilderContextPooledObjectPolicy()),
-            new DefaultInlineConstraintResolver(Options.Create(new RouteOptions()), new TestServiceProvider()));
+            new DefaultInlineConstraintResolver(Options.Create(routeOptions), new TestServiceProvider()));
 
         var selector = new DefaultEndpointSelector();
 

src/Http/Routing/test/UnitTests/RouteTest.cs

@@ -1861,6 +1861,7 @@ private static IInlineConstraintResolver GetInlineConstraintResolver()
     private static void ConfigureRouteOptions(RouteOptions options)
     {
         options.ConstraintMap["test-policy"] = typeof(TestPolicy);
+        options.SetParameterPolicy<RegexInlineRouteConstraint>("regex");
     }
 
     private class TestPolicy : IParameterPolicy