$re = '/^((?!class="icons-background").)*$/';
$str = '<path fill="#20262d" class="icons-background" d="M23.8 0c3.5 6.4 4.2 16-9.8"></path>';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE, 0);
// Print the entire match result
var_dump($matches);
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