diff --git a/app_pojavlauncher/src/main/java/net/kdt/pojavlaunch/fragments/LocalLoginFragment.java b/app_pojavlauncher/src/main/java/net/kdt/pojavlaunch/fragments/LocalLoginFragment.java
index e7c71f326..86a00e375 100644
--- a/app_pojavlauncher/src/main/java/net/kdt/pojavlaunch/fragments/LocalLoginFragment.java
+++ b/app_pojavlauncher/src/main/java/net/kdt/pojavlaunch/fragments/LocalLoginFragment.java
@@ -1,5 +1,6 @@
 package net.kdt.pojavlaunch.fragments;
 
+import android.content.Context;
 import android.os.Bundle;
 import android.view.View;
 import android.widget.EditText;
@@ -20,17 +21,23 @@ import java.util.regex.Pattern;
 public class LocalLoginFragment extends Fragment {
     public static final String TAG = "LOCAL_LOGIN_FRAGMENT";
 
+    private final Pattern mUsernameValidationPattern;
     private EditText mUsernameEditText;
 
     public LocalLoginFragment(){
         super(R.layout.fragment_local_login);
+        mUsernameValidationPattern = Pattern.compile("^[a-zA-Z0-9_]*$");
     }
 
     @Override
     public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
         mUsernameEditText = view.findViewById(R.id.login_edit_email);
         view.findViewById(R.id.login_button).setOnClickListener(v -> {
-            if(!checkEditText()) return;
+            if(!checkEditText()) {
+                Context context = v.getContext();
+                Tools.dialog(context, context.getString(R.string.local_login_bad_username_title), context.getString(R.string.local_login_bad_username_text));
+                return;
+            }
 
             ExtraCore.setValue(ExtraConstants.MOJANG_LOGIN_TODO, new String[]{
                     mUsernameEditText.getText().toString(), "" });
@@ -45,13 +52,11 @@ public class LocalLoginFragment extends Fragment {
 
         String text = mUsernameEditText.getText().toString();
 
-        Pattern pattern = Pattern.compile("[^a-zA-Z0-9_]");
-        Matcher matcher = pattern.matcher(text);
-
+        Matcher matcher = mUsernameValidationPattern.matcher(text);
         return !(text.isEmpty()
                 || text.length() < 3
                 || text.length() > 16
-                || matcher.find()
+                || !matcher.find()
                 || new File(Tools.DIR_ACCOUNT_NEW + "/" + text + ".json").exists()
         );
     }
diff --git a/app_pojavlauncher/src/main/res/values/strings.xml b/app_pojavlauncher/src/main/res/values/strings.xml
index 3fc11ce7a..9e977c1aa 100644
--- a/app_pojavlauncher/src/main/res/values/strings.xml
+++ b/app_pojavlauncher/src/main/res/values/strings.xml
@@ -416,4 +416,6 @@
     <string name="preference_remap_controller_description">Allows you to modify the keyboard keys bound to each controller button</string>
     <string name="mcl_button_discord">Discord</string>
     <string name="discord_invite" translatable="false">https://discord.gg/pojavlauncher-724163890803638273</string>
+    <string name="local_login_bad_username_title">Unsuitable username</string>
+    <string name="local_login_bad_username_text">The username must be between 3–16 characters long, and must only contain latin letters, arabic numerals and underscores.</string>
 </resources>