protected function _compress() {
        //$before = '(?<=[:(, ])';
        //$after = '(?=[ ,);}])';
        //$units = '(em|ex|%|px|cm|mm|in|pt|pc|ch|rem|vh|vw|vmin|vmax|vm)';

        foreach ($this->_data as $value) {
            $this->_concatContent .= $value;
        }

        if ($this->_type == 'css') {
            $this->_concatContent = preg_replace('!/\*[^*]*\*+([^/][^*]*\*+)*/!', '', $this->_concatContent);
            $this->_concatContent = str_replace(': ', ':', $this->_concatContent);
            $this->_concatContent = str_replace(["\r\n", "\r", "\n", "\t", '  ', '    ', '    '], '', $this->_concatContent);
        }
    }