pterodactyl/panel

use Illuminate\Contracts\Validation\Rule;

class Username implements Rule
{
    public const VALIDATION_REGEX = '/^[a-z0-9]([\w\.-]+)[a-z0-9]$/';

    public function passes($attribute, $value): bool
    {
        return preg_match(self::VALIDATION_REGEX, mb_strtolower($value));
    }

    public function message(): string
    {
        return 'The :attribute must start and end with alpha-numeric characters and
                contain only letters, numbers, dashes, underscores, and periods.';
    }

    public function __toString()
    {
        return 'p_username';
    }
}

use Pterodactyl\Rules\Username;

class User extends Model
{
    //
    public static function getRules()
    {
        $rules = parent::getRules();

        $rules['language'][] = new In(array_keys((new self())->getAvailableLanguages()));
        $rules['username'][] = new Username();

        return $rules;
    }
    //
}