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public function rules()
{
    return [
        ['regions', 'validateRegion'],
    ];
}



public function validateRegion($attribute, $params)
{
    $data = $this->$attribute;

    if (!is_array($data) || trim($data[0]) == '') {
        $this->addError($attribute, 'Выберите хотя бы один регион');
    }
}