$re = '/^[^\]]*\]\s+\[(?<my_field>[^\]]*)/m';
$str = ' 2019-10-14 10:25:30,860 [0.16.132.114:8443-78] [/microdc_1]
2019-10-14 10:25:30,854 [0.16.132.114:8443-78] [/microdc_1]
2019-10-14 10:25:30,813 [0.16.132.114:8443-78] [/microdc_1]
2019-10-14 10:25:30,526 [10.16.142.94:8443-75] [TABTHREAD1]
2019-10-14 10:25:30,514 [.16.132.111:8443-146] [/microdc_1]
2019-10-14 10:25:30,467 [.16.136.140:8443-123] [/microdc_2]
2019-10-14 10:25:30,466 [.16.136.140:8443-123] [/microdc_2]
2019-10-14 10:25:30,103 [.16.132.111:8443-146] [/microdc_1]
2019-10-14 10:25:30,097 [.16.132.111:8443-146] [/microdc_1]
2019-10-14 10:25:30,078 [.16.132.111:8443-146] [/microdc_1]
2019-10-14 10:25:29,888 [.16.134.114:8443-128] [/microdc_1]
2019-10-14 10:25:29,883 [.16.134.114:8443-128] [/microdc_1]
2019-10-14 10:25:29,865 [.16.134.114:8443-128] [/microdc_1]
2019-10-14 10:25:29,638 [0.16.130.100:8443-71] [TABTHREAD1]
2019-10-14 10:25:29,594 [10.16.142.97:8443-80] [TABTHREAD2]
2019-10-14 10:25:29,594 [10.16.142.97:8443-80] [TABTHREAD2]
2019-10-14 10:25:29,502 [.16.130.104:8443-144] [TABTHREAD1]
2019-10-14 10:25:29,462 [0.16.134.106:8443-59] [ ]
2019-10-14 10:25:29,337 [0.16.130.100:8443-47] [TABTHREAD1]
2019-10-14 10:25:29,270 [0.16.134.106:8443-59] [TABTHREAD1]';
preg_match_all($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