$re = '/^[^\[]+\[[^\]]+\]\s+\[(?<log_level>[^\]]+)\]\s+\[[^\]]*\]\s+(?<your_field>[^\]]+)/m';
$str = '{"log":"[10:30:04.075] [INFO ] [] [c.c.n.b.i.DefaultBusinessEventService] [akka://MmsAuCluster/system/sharding/notificationAuthBpmn/4/nmT9K3rySjyoHHzxO9jHnQ_4/nmT9K3rySjyoHHzxO9jHnQ] - method=prepare; triggerName=approvalStart, entity={\'id\'=\'0f86c9007ff511ed82ffd13c4d1f79a9a07ff511ed82ffd13c4d173b0a\',\'eventCode\'=\'approval\',\'paymentSystemId\'=\'MMS\',\'servicingAgentBIC\'=\'null\',\'messageIdentification\'=\'0f86ff511ed82ffd13c4d173b0a\',\'businessDomainName\'=\'Mandate\',\'catalogCode\'=\'AN\',\'functionCode\'=\'APAL_INTERACTION\'}';
preg_match_all($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