skeeks\cms\validators\LoginValidator::validateAttribute PHP Method

    public function validateAttribute($model, $attribute)
    {
        $string = $model->{$attribute};
        if (!preg_match('/^[a-z]{1}[a-z0-9_]+$/', $string)) {
            $this->addError($model, $attribute, \Yii::t('skeeks/cms', 'Use only letters (lowercase) and numbers. Must begin with a letter. Example {sample}', ['sample' => 'demo1']));
            return false;
        }
    }