false;
    }

    /**
     * Removes non-printable ASCII characters from string & trims it
     *
     * @param string $value
     * @return bool
     */
    protected function removeNonPrintableCharacters($value)
    {
        return trim(preg_replace('/[^ -~]/xu','',$value));
    }

    /**
     * Does this attribute value have a remote reference?
     *
     * @param $value
     * @return bool
     */
    protected function hasRemoteReference($value)
    {
        $value = $this->removeNonPrintableCharacters($value);

        $wrapped_in_url = preg_match('~^url\(\s*[\'"]\s*(.*)\s*[\'"]\s*\)$~xi', $value, $match);
        if (!$wrapped_in_url){
            return false;
        }

        $value = trim($match[1], '\'"');

        return preg_match('~^((https?|ftp|file):)?//~xi', $value);
    }

    /**
     * Should we minify the output?
     *
     * @param bool $shouldMinify
     */
    public function minify($shouldMinify = false)
    {
        $this->minifyXML = (bool) $shouldMinify;
    }

    /**
     * Should we remove the XML tag in the header?
     *
     * @param bool $removeXMLTag
     */
    public function removeXMLTag($removeXMLTag = false)
    {
        $this->removeXMLTag = (bool) $removeXMLTag;
    }

    /**
     * Whether `<use ... xlink:href="#identifier">` elements shall be
     * removed in case expansion would exceed this threshold.
     *
     * @param int $useThreshold
     */
    public function useThreshold($useThreshold = 1000)
    {
        $this->useThreshold = (int)$useThreshold;
    }

    /**
     * Check to see if an attribute is an aria attribute or not
     *
     * @param $attributeName
     *
     * @return bool
     */
    protected function isAriaAttribute($attributeName)
    {
        return strpos($attributeName, 'aria-') === 0;
    }

    /**
     * Check to see if an attribute is an data attribute or not
     *
     * @param $attributeName
     *
     * @return bool
     */
    protected function isDataAttribute($attributeName)
    {
        return strpos($attributeName, 'data-') === 0;
    }

    /**
     * Make sure our use tag is only referencing internal resources
     *
     * @param \DOMElement $element
     * @return bool
     */
    protected function isUseTagDirty(\DOMElement $element)
    {
        $href = Helper::getElementHref($element);
        return $href && strpos($href, '#') !== 0;
    }

    /**
     * Determines whether `<use ... xlink:href="#identifier">` is expanded
     * recursively in order to create DoS scenarios. The amount of a actually
     * used element needs to be below `$this->useThreshold`.
     *
     * @param \DOMElement $element
     * @return bool
     */
    protected function isUseTagExceedingThreshold(\DOMElement $element)
    {
        if ($this->useThreshold <= 0) {
            return false;
        }
        $useId = Helper::extractIdReferenceFromHref(
            Helper::getElementHref($element)
        );
        if ($useId === null) {
            return false;
        }
        foreach ($this->elementReferenceResolver->findByElementId($useId) as $subject) {
            if ($subject->countUse() >= $this->useThreshold) {
                return true;
            }
        }
        return false;
    }

    /**
     * Set the nesting limit for <use> tags.
     *
     * @param $limit
     */
    public function setUseNestingLimit($limit)
    {
        $this->useNestingLimit = (int) $limit;
    }

    /**
     * Remove nodes that are either invalid or malformed.
     *
     * @param \DOMNode $currentElement The current element.
     */
    protected function cleanUnsafeNodes(\DOMNode $currentElement) {
        // Replace CDATA node with encoded text node
        if ($currentElement instanceof \DOMCdataSection) {
            $textNode = $currentElement->ownerDocument->createTextNode($currentElement->nodeValue);
            $currentElement->parentNode->replaceChild($textNode, $currentElement);
        // If the element doesn't have a tagname, remove it and continue with next iteration
        } elseif (!$currentElement instanceof \DOMElement && !$currentElement instanceof \DOMText) {
            $currentElement->parentNode->removeChild($currentElement);
            $this->xmlIssues[] = array(
                'message' => 'Suspicious node \'' . $currentElement->nodeName . '\'',
                'line' => $currentElement->getLineNo(),
            );
            return;
        }

        if ( $currentElement->childNodes && $currentElement->childNodes->length > 0 ) {
            for ($j = $currentElement->childNodes->length - 1; $j >= 0; $j--) {
                /** @var \DOMElement $childElement */
                $childElement = $currentElement->childNodes->item($j);
                $this->cleanUnsafeNodes($childElement);
            }
        }
    }

    /**
     * Retrieve array of errors
     * @return array
     */
    private static function getXmlErrors()
    {
        $errors = [];
        foreach (libxml_get_errors() as $error) {
            $errors[] = [
                'message' => trim($error->message),
                'line' => $error->line,
            ];
        }

        return $errors;
    }
}