Add specific message for username validation

src/Core/Validator/AbstractValidator.php

@@ -14,7 +14,6 @@ use Flarum\Event\ConfigureValidator;
 use Illuminate\Contracts\Events\Dispatcher;
 use Illuminate\Contracts\Validation\ValidationException;
 use Illuminate\Validation\Factory;
-use Illuminate\Validation\Validator;
 use Symfony\Component\Translation\TranslatorInterface;
 
 abstract class AbstractValidator

src/Core/Validator/UserValidator.php

@@ -61,4 +61,14 @@ class UserValidator extends AbstractValidator
             ]
         ];
     }
+
+    /**
+     * {@inheritdoc}
+     */
+    protected function getMessages()
+    {
+        return [
+            'username.regex' => $this->translator->trans('core.api.invalid_username_message')
+        ];
+    }
 }