fix(library): use conditional regex fallback for older frameworks

Co-authored-by: baywet <7905502+baywet@users.noreply.github.com>

Author: Copilot

agents.md

@@ -0,0 +1,6 @@
+# Regex handling
+
+- For fixed patterns on modern targets, use source-generated regexes with explicit match timeouts (`GeneratedRegex` under `NET8_0_OR_GREATER`).
+- Use conditional compilation to provide a regular `Regex` with the same pattern and an explicit match timeout for older targets. Do not duplicate regex validation with a manually maintained character scanner.
+- Older-runtime regex matching may still time out under load because timeouts use wall-clock time. If consumers encounter this limitation, recommend upgrading to a modern runtime that uses the source-generated implementation.
+- Keep shared patterns in constants and reference those constants in validation diagnostics and tests.

src/Microsoft.OpenApi/Validations/Rules/OpenApiComponentsRules.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license.
 
+using System;
 using System.Collections.Generic;
 using System.Text.RegularExpressions;
 
@@ -20,6 +21,8 @@ public static partial class OpenApiComponentsRules
 #if NET8_0_OR_GREATER
         [GeneratedRegex(KeyPattern, RegexOptions.None, matchTimeoutMilliseconds: 100)]
         private static partial Regex KeyRegex();
+#else
+        private static readonly Regex KeyRegex = new(KeyPattern, RegexOptions.None, TimeSpan.FromMilliseconds(100));
 #endif
 
         /// <summary>
@@ -61,7 +64,7 @@ private static void ValidateKeys(IValidationContext context, IEnumerable<string>
 #if NET8_0_OR_GREATER
                 var isValidKey = KeyRegex().IsMatch(key);
 #else
-                var isValidKey = IsValidKey(key);
+                var isValidKey = KeyRegex.IsMatch(key);
 #endif
                 if (!isValidKey)
                 {
@@ -71,34 +74,5 @@ private static void ValidateKeys(IValidationContext context, IEnumerable<string>
             }
         }
 
-        internal static bool IsValidKey(string key)
-        {
-            // Match the fixed character class without wall-clock timeouts on downlevel targets.
-            var length = key.Length;
-            // The regex's $ anchor also accepts a single final newline.
-            if (length > 0 && key[length - 1] == '\n')
-            {
-                length--;
-            }
-
-            if (length == 0)
-            {
-                return false;
-            }
-
-            for (var i = 0; i < length; i++)
-            {
-                var character = key[i];
-                if (!(character >= 'a' && character <= 'z') &&
-                    !(character >= 'A' && character <= 'Z') &&
-                    !(character >= '0' && character <= '9') &&
-                    character != '.' && character != '-' && character != '_')
-                {
-                    return false;
-                }
-            }
-
-            return true;
-        }
     }
 }

test/Microsoft.OpenApi.Tests/Validations/OpenApiComponentsValidationTests.cs

@@ -36,7 +36,7 @@ public void ValidateKeyMustMatchRegularExpressionInComponents()
             Assert.False(result);
             Assert.NotNull(errors);
             var error = Assert.Single(errors);
-            Assert.Equal(string.Format(SRResource.Validation_ComponentsKeyMustMatchRegularExpr, key, "responses", @"^[a-zA-Z0-9\.\-_]+$"),
+            Assert.Equal(string.Format(SRResource.Validation_ComponentsKeyMustMatchRegularExpr, key, "responses", OpenApiComponentsRules.KeyPattern),
                 error.Message);
         }
 
@@ -67,7 +67,6 @@ public void ValidateComponentKeyPreservesRegexBehavior(string key, bool isValid)
             rules.Add(typeof(OpenApiComponents), OpenApiComponentsRules.KeyMustBeRegularExpression);
             var errors = components.Validate(rules);
 
-            Assert.Equal(isValid, OpenApiComponentsRules.IsValidKey(key));
             Assert.Equal(isValid, !errors.Any());
         }
 
@@ -77,7 +76,6 @@ public void ValidateComponentKeyPreservesRegexBehavior(string key, bool isValid)
         public async Task LoadAsyncValidatesLongComponentKeys(bool isValid)
         {
             var key = new string('a', 1_000_000) + (isValid ? string.Empty : "!");
-            Assert.Equal(isValid, OpenApiComponentsRules.IsValidKey(key));
             var json = """
                 {"openapi":"3.1.0","info":{"title":"Test","version":"1.0"},"paths":{},"components":{"schemas":{
                 """ + JsonSerializer.Serialize(key) + ":{\"type\":\"string\"}}}}";
@@ -95,7 +93,7 @@ public async Task LoadAsyncValidatesLongComponentKeys(bool isValid)
             {
                 var error = Assert.Single(result.Diagnostic.Errors);
                 Assert.Equal(string.Format(SRResource.Validation_ComponentsKeyMustMatchRegularExpr,
-                    key, "schemas", @"^[a-zA-Z0-9\.\-_]+$"), error.Message);
+                    key, "schemas", OpenApiComponentsRules.KeyPattern), error.Message);
             }
         }
     }