Fix Emote Chat Sanitizer (#32940)

* Fix bug?

* Fix :)

* aaaa

* AAAA!!!

* comment

* Nicer code

* What's a pull requestWhat's a pull request?

Content.Shared/Chat/SharedChatSystem.cs

@@ -84,6 +84,35 @@ public abstract class SharedChatSystem : EntitySystem
         return current ?? _prototypeManager.Index<SpeechVerbPrototype>(speech.SpeechVerb);
     }
 
+    /// <summary>
+    /// Splits the input message into a radio prefix part and the rest to preserve it during sanitization.
+    /// </summary>
+    /// <remarks>
+    /// This is primarily for the chat emote sanitizer, which can match against ":b" as an emote, which is a valid radio keycode.
+    /// </remarks>
+    public void GetRadioKeycodePrefix(EntityUid source,
+        string input,
+        out string output,
+        out string prefix)
+    {
+        prefix = string.Empty;
+        output = input;
+
+        // If the string is less than 2, then it's probably supposed to be an emote.
+        // No one is sending empty radio messages!
+        if (input.Length <= 2)
+            return;
+
+        if (!(input.StartsWith(RadioChannelPrefix) || input.StartsWith(RadioChannelAltPrefix)))
+            return;
+
+        if (!_keyCodes.TryGetValue(input[1], out _))
+            return;
+
+        prefix = input[..2];
+        output = input[2..];
+    }
+
     /// <summary>
     ///     Attempts to resolve radio prefixes in chat messages (e.g., remove a leading ":e" and resolve the requested
     ///     channel. Returns true if a radio message was attempted, even if the channel is invalid.