Stanislav

$config = [
    // ...
    'params' => require(__DIR__ . '/params.php'),

    // redirect to the page with the trailing slash
    'on beforeRequest' => function () {
        $app = Yii::$app;
        $pathInfo = $app->request->pathInfo;
        if (!empty($pathInfo) && substr($pathInfo, -1) !== '/') {
            $app->response->redirect('/' . rtrim($pathInfo) . '/', 301);
        }
    },
];

public function rules()
    {
        return [
            [
                'user_name', 
                'match', 
                'pattern'=>'/^[A-Z][A-Za-z]*_[A-Z][A-Za-z]*$/', 
            ],
        ];
    }