diff --git a/agents.md b/agents.md
new file mode 100644
index 000000000..abeed736e
--- /dev/null
+++ b/agents.md
@@ -0,0 +1,7 @@
+# Regex handling
+
+- Analyze regex patterns for matching complexity and excessive backtracking, including on long and near-matching inputs. Prefer equivalent patterns with less backtracking when available, while preserving matching semantics and target-framework compatibility.
+- 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.
diff --git a/src/Microsoft.OpenApi/Expressions/CompositeExpression.cs b/src/Microsoft.OpenApi/Expressions/CompositeExpression.cs
index cac554318..db2a97caf 100644
--- a/src/Microsoft.OpenApi/Expressions/CompositeExpression.cs
+++ b/src/Microsoft.OpenApi/Expressions/CompositeExpression.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.Linq;
 using System.Text.RegularExpressions;
@@ -10,10 +11,17 @@ namespace Microsoft.OpenApi
     /// <summary>
     /// String literal with embedded expressions
     /// </summary>
-    public class CompositeExpression : RuntimeExpression
+    public partial class CompositeExpression : RuntimeExpression
     {
         private readonly string template;
-        private readonly Regex expressionPattern = new(@"{(?<exp>\$[^}]*)");
+        private const string ExpressionPattern = @"{(?<exp>\$[^}]*)";
+
+#if NET8_0_OR_GREATER
+        [GeneratedRegex(ExpressionPattern, RegexOptions.None, matchTimeoutMilliseconds: 100)]
+        private static partial Regex ExpressionRegex();
+#else
+        private static readonly Regex ExpressionRegex = new(ExpressionPattern, RegexOptions.None, TimeSpan.FromMilliseconds(100));
+#endif
 
         /// <summary>
         /// Expressions embedded into string literal
@@ -24,12 +32,17 @@ public class CompositeExpression : RuntimeExpression
         /// Create a composite expression from a string literal with an embedded expression
         /// </summary>
         /// <param name="expression"></param>
+        /// <exception cref="RegexMatchTimeoutException">Extracting embedded expressions exceeds the regex match timeout.</exception>
         public CompositeExpression(string expression)
         {
             template = expression;
 
             // Extract subexpressions and convert to RuntimeExpressions
-            var matches = expressionPattern.Matches(expression);
+#if NET8_0_OR_GREATER
+            var matches = ExpressionRegex().Matches(expression);
+#else
+            var matches = ExpressionRegex.Matches(expression);
+#endif
 
             foreach (var item in matches.Cast<Match>())
             {
diff --git a/src/Microsoft.OpenApi/Validations/Rules/OpenApiComponentsRules.cs b/src/Microsoft.OpenApi/Validations/Rules/OpenApiComponentsRules.cs
index e3c6e1b4f..7473bce91 100644
--- a/src/Microsoft.OpenApi/Validations/Rules/OpenApiComponentsRules.cs
+++ b/src/Microsoft.OpenApi/Validations/Rules/OpenApiComponentsRules.cs
@@ -11,12 +11,19 @@ namespace Microsoft.OpenApi
     /// The validation rules for <see cref="OpenApiComponents"/>.
     /// </summary>
     [OpenApiRule]
-    public static class OpenApiComponentsRules
+    public static partial class OpenApiComponentsRules
     {
         /// <summary>
-        /// The key regex.
+        /// The key regex pattern.
         /// </summary>
-        internal static readonly Regex KeyRegex = new(@"^[a-zA-Z0-9\.\-_]+$", RegexOptions.None, TimeSpan.FromMilliseconds(100));
+        internal const string KeyPattern = @"^[a-zA-Z0-9\.\-_]+$";
+
+#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>
         /// All the fixed fields declared above are objects
@@ -54,12 +61,18 @@ private static void ValidateKeys(IValidationContext context, IEnumerable<string>
 
             foreach (var key in keys)
             {
-                if (!KeyRegex.IsMatch(key))
+#if NET8_0_OR_GREATER
+                var isValidKey = KeyRegex().IsMatch(key);
+#else
+                var isValidKey = KeyRegex.IsMatch(key);
+#endif
+                if (!isValidKey)
                 {
                     context.CreateError(nameof(KeyMustBeRegularExpression),
-                        string.Format(SRResource.Validation_ComponentsKeyMustMatchRegularExpr, key, component, KeyRegex.ToString()));
+                        string.Format(SRResource.Validation_ComponentsKeyMustMatchRegularExpr, key, component, KeyPattern));
                 }
             }
         }
+
     }
 }
diff --git a/test/Microsoft.OpenApi.Tests/Expressions/RuntimeExpressionTests.cs b/test/Microsoft.OpenApi.Tests/Expressions/RuntimeExpressionTests.cs
index 08a1debc0..91509184b 100644
--- a/test/Microsoft.OpenApi.Tests/Expressions/RuntimeExpressionTests.cs
+++ b/test/Microsoft.OpenApi.Tests/Expressions/RuntimeExpressionTests.cs
@@ -145,6 +145,28 @@ public void BuildRuntimeExpressionTwiceCreatesNewEquivalentInstances(string expr
             Assert.Equal(runtimeExpression1, runtimeExpression2);
         }
 
+        [Fact]
+        public void CompositeRuntimeExpressionPreservesMultilineCaptures()
+        {
+            const string expression = "prefix {$request.header.foo\nbar} {$url} suffix";
+
+            var composite = Assert.IsType<CompositeExpression>(RuntimeExpression.Build(expression));
+
+            Assert.Equal(expression, composite.Expression);
+            Assert.Equal(new[] { "$request.header.foo\nbar", "$url" },
+                composite.ContainedExpressions.Select(static item => item.Expression));
+        }
+
+        [Fact]
+        public void CompositeRuntimeExpressionPreservesUnterminatedCapture()
+        {
+            const string expression = "prefix {$url";
+
+            var composite = Assert.IsType<CompositeExpression>(RuntimeExpression.Build(expression));
+
+            Assert.IsType<UrlExpression>(Assert.Single(composite.ContainedExpressions));
+        }
+
         [Fact]
         public void CompositeRuntimeExpressionContainsExpression()
         {
diff --git a/test/Microsoft.OpenApi.Tests/Validations/OpenApiComponentsValidationTests.cs b/test/Microsoft.OpenApi.Tests/Validations/OpenApiComponentsValidationTests.cs
index 68f89a2a2..8993c17a5 100644
--- a/test/Microsoft.OpenApi.Tests/Validations/OpenApiComponentsValidationTests.cs
+++ b/test/Microsoft.OpenApi.Tests/Validations/OpenApiComponentsValidationTests.cs
@@ -2,7 +2,11 @@
 // Licensed under the MIT license.
 
 using System.Collections.Generic;
+using System.IO;
 using System.Linq;
+using System.Text;
+using System.Text.Json;
+using System.Threading.Tasks;
 using Xunit;
 
 namespace Microsoft.OpenApi.Validations.Tests
@@ -32,8 +36,65 @@ public void ValidateKeyMustMatchRegularExpressionInComponents()
             Assert.False(result);
             Assert.NotNull(errors);
             var error = Assert.Single(errors);
-            Assert.Equal(string.Format(SRResource.Validation_ComponentsKeyMustMatchRegularExpr, key, "responses", OpenApiComponentsRules.KeyRegex.ToString()),
+            Assert.Equal(string.Format(SRResource.Validation_ComponentsKeyMustMatchRegularExpr, key, "responses", OpenApiComponentsRules.KeyPattern),
                 error.Message);
         }
+
+        [Theory]
+        [InlineData("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789.-_", true)]
+        [InlineData("", false)]
+        [InlineData("a b", false)]
+        [InlineData("a/b", false)]
+        [InlineData("é", false)]
+        [InlineData("１２", false)]
+        [InlineData("a\n", true)]
+        [InlineData("\n", false)]
+        [InlineData("a\n\n", false)]
+        [InlineData("a\r\n", false)]
+        [InlineData("a\nb", false)]
+        [InlineData("a\0", false)]
+        public void ValidateComponentKeyPreservesRegexBehavior(string key, bool isValid)
+        {
+            var components = new OpenApiComponents
+            {
+                Schemas = new Dictionary<string, IOpenApiSchema>
+                {
+                    { key, new OpenApiSchema() }
+                }
+            };
+
+            var rules = new ValidationRuleSet();
+            rules.Add(typeof(OpenApiComponents), OpenApiComponentsRules.KeyMustBeRegularExpression);
+            var errors = components.Validate(rules);
+
+            Assert.Equal(isValid, !errors.Any());
+        }
+
+        [Theory]
+        [InlineData(true)]
+        [InlineData(false)]
+        public async Task LoadAsyncValidatesLongComponentKeys(bool isValid)
+        {
+            var key = new string('a', 1_000_000) + (isValid ? string.Empty : "!");
+            var json = """
+                {"openapi":"3.1.0","info":{"title":"Test","version":"1.0"},"paths":{},"components":{"schemas":{
+                """ + JsonSerializer.Serialize(key) + ":{\"type\":\"string\"}}}}";
+            using var stream = new MemoryStream(Encoding.UTF8.GetBytes(json));
+
+            var result = await OpenApiDocument.LoadAsync(stream, cancellationToken: TestContext.Current.CancellationToken);
+
+            Assert.NotNull(result.Document);
+            Assert.NotNull(result.Diagnostic);
+            if (isValid)
+            {
+                Assert.Empty(result.Diagnostic.Errors);
+            }
+            else
+            {
+                var error = Assert.Single(result.Diagnostic.Errors);
+                Assert.Equal(string.Format(SRResource.Validation_ComponentsKeyMustMatchRegularExpr,
+                    key, "schemas", OpenApiComponentsRules.KeyPattern), error.Message);
+            }
+        }
     }
 }