$re = '/^\[[^\]]+\]\s+\[(?<firstfieldname>[^\]]+)\]\s+\[(?<secondfieldname>[^\]]+)\]\s+\[(?<thirdfieldname>[^\]]+)\]\s+\[(?<fourthfieldname>[^\]]+)\]\s+\[(?<username>[^\]]+)\]\s+(?<message>.+)/m';
$str = '[2020-03-29 18:37:51,020] [IG Audit Writer] [INFO ] [IG.AUDIT] [AUD6012] [UserNameHere] EventMessageHere';
preg_match_all($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