$re = '/((?=<[^>]>)[^;]+;[^;]+;|<[^>]>|<\/[^>]+>|<[^\'"]+[\'"]|[\'"][^<]+<[^>]+>)/m';
$str = 'Customers are advised to follow <A HREF=\'https://support.microsoft.com/en-ph/help/4343902/security-update-for-adobe-flash-player\' TARGET=\'_blank\'>KB4343902</A> for instructions pertaining to the remediation of these vulnerabilities.<P> <P>Patch:<br/> Following are links for downloading patches to fix the vulnerabilities: <P> <A HREF=\'https://portal.msrc.microsoft.com/en-us/security-guidance/advisory/ADV180020\' TARGET=\'_blank\'>ADV180020</A>';
$subst = "";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php