$re = '/Cluster\[(?<string1>[^\.]+)\.(?<string2>[^\]]+)/m';
$str = '2022-02-01 11:59:59,869 INFO CUS.AbCD-Host-000000 [AppListener] Receive Packet[0000000*]: Cluster[String1.String2]';
preg_match_all($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