diff --git a/csharp/src/Google.Protobuf/Reflection/DescriptorPool.cs b/csharp/src/Google.Protobuf/Reflection/DescriptorPool.cs
index 5128ad9a6a83..3956b55e353d 100644
--- a/csharp/src/Google.Protobuf/Reflection/DescriptorPool.cs
+++ b/csharp/src/Google.Protobuf/Reflection/DescriptorPool.cs
@@ -33,7 +33,6 @@
 using System;
 using System.Collections.Generic;
 using System.Text;
-using System.Text.RegularExpressions;
 
 namespace Google.Protobuf.Reflection
 {
@@ -176,8 +175,6 @@ internal void AddSymbol(IDescriptor descriptor)
             descriptorsByName[fullName] = descriptor;
         }
 
-        private static readonly Regex ValidationRegex = new Regex("^[_A-Za-z][_A-Za-z0-9]*$", FrameworkPortability.CompiledRegexWhereAvailable);
-
         /// <summary>
         /// Verifies that the descriptor's name is valid (i.e. it contains
         /// only letters, digits and underscores, and does not start with a digit).
@@ -189,11 +186,24 @@ private static void ValidateSymbolName(IDescriptor descriptor)
             {
                 throw new DescriptorValidationException(descriptor, "Missing name.");
             }
-            if (!ValidationRegex.IsMatch(descriptor.Name))
-            {
-                throw new DescriptorValidationException(descriptor,
-                                                        "\"" + descriptor.Name + "\" is not a valid identifier.");
+
+            // Symbol name must start with a letter or underscore, and it can contain letters,
+            // numbers and underscores.
+            string name = descriptor.Name;
+            if (!IsAsciiLetter(name[0]) && name[0] != '_') {
+                ThrowInvalidSymbolNameException(descriptor);
             }
+            for (int i = 1; i < name.Length; i++) {
+                if (!IsAsciiLetter(name[i]) && !IsAsciiDigit(name[i]) && name[i] != '_') {
+                    ThrowInvalidSymbolNameException(descriptor);
+                }
+            }
+
+            static bool IsAsciiLetter(char c) => (uint)((c | 0x20) - 'a') <= 'z' - 'a';
+            static bool IsAsciiDigit(char c) => (uint)(c - '0') <= '9' - '0';
+            static void ThrowInvalidSymbolNameException(IDescriptor descriptor) =>
+                throw new DescriptorValidationException(
+                    descriptor, "\"" + descriptor.Name + "\" is not a valid identifier.");
         }
 
         /// <summary>
diff --git a/objectivec/GPBAny.pbobjc.h b/objectivec/GPBAny.pbobjc.h
index f4efbb23be2c..c0d389f0399c 100644
--- a/objectivec/GPBAny.pbobjc.h
+++ b/objectivec/GPBAny.pbobjc.h
@@ -157,7 +157,8 @@ GPB_FINAL @interface GPBAny : GPBMessage
  *
  * Note: this functionality is not currently available in the official
  * protobuf release, and it is not used for type URLs beginning with
- * type.googleapis.com.
+ * type.googleapis.com. As of May 2023, there are no widely used type server
+ * implementations and no plans to implement one.
  *
  * Schemes other than `http`, `https` (or the empty scheme) might be
  * used with implementation specific semantics.