$re = '`.*\[([^\]]*)\].*`m';
$str = 'Zabbix: disk_avgerage[dae_1_0_disk_2]
Zabbix: disk_avgerage[dae_4_4_disk_1]
Zabbix: disk_avgerage[dae_5_1_disk_1]
Zabbix: disk_avgerage[dae_4_4_disk_9]
Zabbix: disk_avgerage[dae_5_0_disk_3]';
preg_match_all($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