$re = '/^(\d{8}_\d{7}_)(?:CUPS[^_]+_)?(\S+)$/m';
$str = '12345678_1234567_wgewgg_ewgewgw_gwgw_gwg
12345678_1234567_CUPS-Error_hgerhrh_herhr_herhe_heh';
$subst = "$1CUPS-ERROR_$2";
$result = preg_replace($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