Client `PatternSyntaxException` on joining server with plugin

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Using:

• Server:
  • Minecraft 1.21.1
  • Purpur build 2329
  • DHS 0.6.1 (from the Google Drive)
  • Multiverse with various worlds (happens on all)
  • various plugins
• Client:
  • Minecraft 1.21.1
  • Fabric Loader 0.16.3
  • DH 2.3.0-b-dev c326e0ba (manually built)

On joining my Purpur server with the DHS plugin and DH 2.3.0-b-dev (commit c326e0ba, latest from main), the following messages are printed:

[XX:55:22] [Render thread/ERROR]: Failed to handle the message. New messages will be ignored.
java.util.regex.PatternSyntaxException: Unclosed counted closure near index 8
^(?=.{1,$MAX_LENGTH}$)([$PART_ALLOWED_CHARS_REGEX]+@)?[$PART_ALLOWED_CHARS_REGEX]+(:[$PART_ALLOWED_CHARS_REGEX]+)?$
        ^
	at java.base/java.util.regex.Pattern.error(Pattern.java:2204) ~[?:?]
	at java.base/java.util.regex.Pattern.closure(Pattern.java:3480) ~[?:?]
	at java.base/java.util.regex.Pattern.sequence(Pattern.java:2390) ~[?:?]
	at java.base/java.util.regex.Pattern.expr(Pattern.java:2245) ~[?:?]
	at java.base/java.util.regex.Pattern.group0(Pattern.java:3152) ~[?:?]
	at java.base/java.util.regex.Pattern.sequence(Pattern.java:2300) ~[?:?]
	at java.base/java.util.regex.Pattern.expr(Pattern.java:2245) ~[?:?]
	at java.base/java.util.regex.Pattern.compile(Pattern.java:1945) ~[?:?]
	at java.base/java.util.regex.Pattern.<init>(Pattern.java:1576) ~[?:?]
	at java.base/java.util.regex.Pattern.compile(Pattern.java:1101) ~[?:?]
	at java.base/java.util.regex.Pattern.matches(Pattern.java:1220) ~[?:?]
	at java.base/java.lang.String.matches(String.java:2969) ~[?:?]
	at com.seibel.distanthorizons.core.api.internal.ClientPluginChannelApi.onLevelInitMessage(ClientPluginChannelApi.java:78) ~[DistantHorizons-2.3.0-b-dev-1.21.1-fabric-neoforge.jar:?]
	at com.seibel.distanthorizons.core.network.event.AbstractNetworkEventSource.lambda$registerHandler$3(AbstractNetworkEventSource.java:149) ~[DistantHorizons-2.3.0-b-dev-1.21.1-fabric-neoforge.jar:?]
	at com.seibel.distanthorizons.core.network.event.AbstractNetworkEventSource.handleMessage(AbstractNetworkEventSource.java:85) ~[DistantHorizons-2.3.0-b-dev-1.21.1-fabric-neoforge.jar:?]
	at com.seibel.distanthorizons.core.network.session.NetworkSession.tryHandleMessage(NetworkSession.java:88) ~[DistantHorizons-2.3.0-b-dev-1.21.1-fabric-neoforge.jar:?]
	at com.seibel.distanthorizons.core.api.internal.ClientApi.pluginMessageReceived(ClientApi.java:369) ~[DistantHorizons-2.3.0-b-dev-1.21.1-fabric-neoforge.jar:?]
	at com.seibel.distanthorizons.fabric.FabricClientProxy.lambda$registerEvents$10(FabricClientProxy.java:313) ~[DistantHorizons-2.3.0-b-dev-1.21.1-fabric-neoforge.jar:?]
	at net.fabricmc.fabric.impl.networking.client.ClientPlayNetworkAddon.lambda$receive$0(ClientPlayNetworkAddon.java:73) ~[fabric-networking-api-v1-4.3.0+c7469b2119-f0b0156a0085be9.jar:?]
	at net.minecraft.class_1255.method_18859(class_1255.java:162) [client-intermediary.jar:?]
	at net.minecraft.class_4093.method_18859(class_4093.java:23) [client-intermediary.jar:?]
	at net.minecraft.class_1255.method_16075(class_1255.java:136) [client-intermediary.jar:?]
	at net.minecraft.class_1255.method_5383(class_1255.java:121) [client-intermediary.jar:?]
	at net.minecraft.class_310.method_1523(class_310.java:1240) [client-intermediary.jar:?]
	at net.minecraft.class_310.method_1514(class_310.java:882) [client-intermediary.jar:?]
	at net.minecraft.client.main.Main.main(Main.java:256) [client-intermediary.jar:?]
	at net.fabricmc.loader.impl.game.minecraft.MinecraftGameProvider.launch(MinecraftGameProvider.java:480) [fabric-loader-0.16.3.jar:?]
	at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:74) [fabric-loader-0.16.3.jar:?]
	at net.fabricmc.loader.impl.launch.knot.KnotClient.main(KnotClient.java:23) [fabric-loader-0.16.3.jar:?]
	at org.prismlauncher.launcher.impl.StandardLauncher.launch(StandardLauncher.java:105) [NewLaunch.jar:?]
	at org.prismlauncher.EntryPoint.listen(EntryPoint.java:129) [NewLaunch.jar:?]
	at org.prismlauncher.EntryPoint.main(EntryPoint.java:70) [NewLaunch.jar:?]
[XX:55:22] [Render thread/INFO]: [CHAT] [Distant Horizons] §4§l§uERROR:§r Failed to handle the message. New messages will be ignored.\njava.util.regex. < ... the same as above, just printed to in-game chat ... >
[XX:55:22] [Render thread/ERROR]: Message: [LevelInitMessage{levelKey=world, serverTime=1731952523588}]
[XX:55:22] [Render thread/INFO]: [CHAT] [Distant Horizons] §4§l§uERROR:§r Message: [LevelInitMessage{levelKey=world, serverTime=1731952523588}]
[XX:55:22] [Render thread/WARN]: Unhandled message: [SessionConfigMessage{config=SessionConfig{values={renderDistanceRadius=256, enableDistantGeneration=true, generationRequestRateLimit=20, enableRealTimeUpdates=false, synchronizeOnLoad=true, syncOnLoadRateLimit=50, maxDataTransferSpeed=0}}}].

It seems that this regex is not being templated at all when it is used, though it doesn't seem to resemble any form of java string templating.

DistantHorizons.toml