Make the npc prefix check less strict and more precise

Towny/src/main/java/com/palmergames/bukkit/towny/listeners/TownyLoginListener.java

@@ -9,6 +9,7 @@
 import java.util.HashSet;
 import java.util.Locale;
 import java.util.Set;
+import java.util.regex.Pattern;
 
 import org.bukkit.NamespacedKey;
 import org.bukkit.event.EventHandler;
@@ -29,9 +30,7 @@ public TownyLoginListener() {
 	public void onPlayerLogin(AsyncPlayerPreLoginEvent event) {
 
 		final String logInName = event.getName();
-		boolean disallowed = isServerAccount(logInName) || isGovernmentAccount(logInName);
-
-		disallowed |= logInName.toLowerCase(Locale.ROOT).startsWith(TownySettings.getNPCPrefix().toLowerCase(Locale.ROOT));
+		boolean disallowed = isServerAccount(logInName) || isGovernmentAccount(logInName) || isPossibleNPCName(logInName);
 
 		if (!disallowed)
 			return;
@@ -57,6 +56,20 @@ private boolean isNationBank(String logInName) {
 		return isDisallowedName(logInName, TownySettings.getNationAccountPrefix());
 	}
 
+	static boolean isPossibleNPCName(String loginName) {
+		final String npcPrefix = TownySettings.getNPCPrefix();
+		if (!loginName.toLowerCase(Locale.ROOT).startsWith(npcPrefix.toLowerCase(Locale.ROOT)))
+			return false;
+
+		final String sub = loginName.substring(npcPrefix.length());
+		if (sub.isEmpty())
+			return false;
+
+		// A npc name can only consist of npc prefix + digits
+		final Pattern onlyDigits = Pattern.compile("^\\d+$");
+		return onlyDigits.matcher(sub).find();
+	}
+
 	private boolean isDisallowedName(String logInName, String disallowedName) {
 		return logInName.startsWith(disallowedName) || logInName.startsWith(disallowedName.replace("-", "_")); 
 	}

Towny/src/test/java/com/palmergames/bukkit/towny/listeners/LoginListenerTests.java

@@ -0,0 +1,30 @@
+package com.palmergames.bukkit.towny.listeners;
+
+import be.seeseemelk.mockbukkit.MockBukkit;
+import com.palmergames.bukkit.towny.TownySettings;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+
+import static com.palmergames.bukkit.towny.listeners.TownyLoginListener.isPossibleNPCName;
+import static org.junit.jupiter.api.Assertions.*;
+
+public class LoginListenerTests {
+	@BeforeAll
+	public static void init() {
+		MockBukkit.getOrCreateMock();
+		TownySettings.loadDefaultConfig();
+	}
+
+	@Test
+	void testNpcPrefix() {
+		final String npcPrefix = TownySettings.getNPCPrefix();
+
+		assertFalse(isPossibleNPCName(npcPrefix)); // The npc prefix itself is not a possible npc name
+		assertTrue(isPossibleNPCName(npcPrefix + "1"));
+		assertTrue(isPossibleNPCName(npcPrefix + "99999999"));
+
+		assertFalse(isPossibleNPCName(npcPrefix + "A"));
+		assertFalse(isPossibleNPCName(npcPrefix + "1A"));
+		assertFalse(isPossibleNPCName("name"));
+	}
+}