$re = '/^\[([^\[]+\[){6}[^:]+:(?<id>[^\@]+)@(?<host>\S+)/m';
$str = '[01/05/2020 13:06:21] BAUAJM_I_30031 Client [CA WAAE Auto:15515][5][abc@hostname.com:50019:12.304.593.10] [0x80c91100][02/05/2020 13:06:21.8474][0:rmk123@abc.bbb.com@hostname.com 0] API ID [34] execution started.
[01/05/2020 13:06:21] BAUAJM_I_30032 Client [CA WAAE Auto:15509][5][bdc.ab.cpm@hostname.com:12345:19.304.293.10] [0x28bbbfff][02/05/2020 13:06:21.6946][0:rmc143@bdc.ab.cpm@hostname.com 0] API ID [66] execution completed. Total time: 0.132519 seconds.';
preg_match_all($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