$re = '/.*\]\s*(?<msg>[^:]+)/m';
$str = '][ERROR][pub-#32738][AssociationRemoteProcessor] Exception while running association: javax.cache.CacheException: class org.apache.ignite.IgniteInterrup
[2023-11-09T06:06:02,015][ERROR][pub-#19230][FedPledgingFlaggingRemoteProcessor] No rejection criteria found for the specified key: CO.';
preg_match_all($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