Bukkit

Not sure what causes it, but once in a while I would check server logs and would see this exception:

22:11:32 [SEVERE] [Commander] An exception was caught during chat replacement processing! Chat passed through.
java.lang.IllegalArgumentException: Illegal group reference
        at java.util.regex.Matcher.appendReplacement(Matcher.java:808)
        at org.digiplex.bukkitplugin.commander.module.PlayerChatModule.onPlayerChat(PlayerChatModule.java:78)
        at sun.reflect.GeneratedMethodAccessor191.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:601)
        at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:330)
        at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62)
        at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:477)
        at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:459)
        at net.minecraft.server.NetServerHandler.chat(NetServerHandler.java:830)
        at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:807)
        at net.minecraft.server.Packet3Chat.handle(Packet3Chat.java:44)
        at net.minecraft.server.NetworkManager.i(NetworkManager.java:216)
        at net.minecraft.server.NetworkManager.c(NetworkManager.java:331)
        at net.minecraft.server.NetworkReaderThread.run(SourceFile:93)

Need more information: What version are you using? See if you can reproduce the error message. What replacement causes this? Are you using a dollar sign ($) anywhere in the replacement?

@tustin2121: Go

Actually, I am using a dollar sign in one of the replacements, I'll change that and see if it still throws the exception.

You will need to put a backslash (\) in front of the dollar sign (\$) to escape the back reference. I will make the error message more user-friendly and/or have the pre-regex replacement code catch an arbitrary character after a dollar sign.