Add enum flag pattern regex to Json schema

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/EnumConverter.cs

@@ -10,6 +10,7 @@
 using System.Text.Encodings.Web;
 using System.Text.Json.Nodes;
 using System.Text.Json.Schema;
+using System.Text.RegularExpressions;
 
 namespace System.Text.Json.Serialization.Converters
 {
@@ -495,8 +496,30 @@ private bool IsDefinedValueOrCombinationOfValues(ulong key)
 
                 if (s_isFlagsEnum)
                 {
-                    // Do not report enum values in case of flags.
-                    return new() { Type = JsonSchemaType.String };
+                    string? pattern = null;
+
+                    if (_enumFieldInfo.Length > 0)
+                    {
+                        using ValueStringBuilder sb = new(stackalloc char[JsonConstants.StackallocCharThreshold]);
+                        sb.Append("^(");
+                        sb.Append(Regex.Escape(_enumFieldInfo[0].JsonName));
+                        for (int i = 1; i < _enumFieldInfo.Length; i++)
+                        {
+                            sb.Append('|');
+                            sb.Append(Regex.Escape(_enumFieldInfo[i].JsonName));
+                        }
+                        sb.Append(")(\\w*,\\w*(");
+                        sb.Append(Regex.Escape(_enumFieldInfo[0].JsonName));
+                        for (int i = 1; i < _enumFieldInfo.Length; i++)
+                        {
+                            sb.Append('|');
+                            sb.Append(Regex.Escape(_enumFieldInfo[i].JsonName));
+                        }
+                        sb.Append("))*$");
+                        pattern = sb.ToString();
+                    }
+
+                    return new() { Type = JsonSchemaType.String, Pattern = pattern };
                 }
 
                 JsonArray enumValues = [];

src/libraries/System.Text.Json/tests/Common/JsonSchemaExporterTests.TestTypes.cs

@@ -92,7 +92,10 @@ public static IEnumerable<ITestData> GetTestDataCore()
             // Enum types
             yield return new TestData<IntEnum>(IntEnum.A, ExpectedJsonSchema: """{"type":"integer"}""");
             yield return new TestData<StringEnum>(StringEnum.A, ExpectedJsonSchema: """{"enum":["A","B","C"]}""");
-            yield return new TestData<FlagsStringEnum>(FlagsStringEnum.A, ExpectedJsonSchema: """{"type":"string"}""");
+            yield return new TestData<FlagsStringEnum>(FlagsStringEnum.A, ExpectedJsonSchema: """{"type":"string","pattern":"^(A|B|C)(\\w*,\\w*(A|B|C))*$"}""");
+            yield return new TestData<FlagsEnumWithNameAttributes>(
+                FlagsEnumWithNameAttributes.NormalFlag,
+                ExpectedJsonSchema: """{"type":"string","pattern":"^(A|\"\\.\\\\)(\\w*,\\w*(A|\"\\.\\\\))*$"}""");
             yield return new TestData<EnumWithNameAttributes>(
                 EnumWithNameAttributes.Value1,
                 AdditionalValues: [EnumWithNameAttributes.Value2],
@@ -1297,6 +1300,15 @@ public enum StringEnum { A, B, C };
         [Flags, JsonConverter(typeof(JsonStringEnumConverter<FlagsStringEnum>))]
         public enum FlagsStringEnum { A = 1, B = 2, C = 4 };
 
+        [Flags, JsonConverter(typeof(JsonStringEnumConverter<FlagsEnumWithNameAttributes>))]
+        public enum FlagsEnumWithNameAttributes
+        {
+            [JsonStringEnumMemberName("A")]
+            NormalFlag = 1,
+            [JsonStringEnumMemberName("\".\\")]
+            StrangeFlag = 2,
+        }
+
         [JsonConverter(typeof(JsonStringEnumConverter<EnumWithNameAttributes>))]
         public enum EnumWithNameAttributes
         {

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/JsonSchemaExporterTests.cs

@@ -67,6 +67,7 @@ public sealed partial class JsonSchemaExporterTests_SourceGen()
         [JsonSerializable(typeof(IntEnum))]
         [JsonSerializable(typeof(StringEnum))]
         [JsonSerializable(typeof(FlagsStringEnum))]
+        [JsonSerializable(typeof(FlagsEnumWithNameAttributes))]
         [JsonSerializable(typeof(EnumWithNameAttributes))]
         // Nullable<T> types
         [JsonSerializable(typeof(bool?))]