$re = '/error(?!s=)/mi';
$str = '1 sample text, more text, errors=123456 more text more text
2 drops=0 link status good errors=0 adapter
3 Error: process failed to start
4 process [ERROR] failed
5 this line might have both ERROR and error=5555 and more text
6 there might be a line that has error=0x02343329 ERROR and more text';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($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