@karthi25, if you are extracting eventId to filter "event123" from your _raw events then rex field extraction is not required. You can filter directly in your base search using "\"eventid\":\"event123\""
Following is a run anywhere search based on the sample data provided which extracts the payload data
| makeresults
| eval _raw="2018-03-02T17:02:27.453185+00:00 ESP-Finance-NPE.development.abctestprocessor-dev a36c4e54-dc5a-4d23-afb3-10f1661b19b4[[APP/PROC/WEB/0]]: cf_foundation=*** cf_app_name=*** cf_app_id=a36c4e54-dc5a-4d23-afb3-10f1661b19b4 cf_org_name=**** cf_org_id=*** cf_space_name=development cf_space_id=*** .source.s_cf_apps 2018-03-02 09:02:27.452 ERROR 14 --- [TaskExecutor-82] c.tmobile.finance.service.LoggerService : {\"host_endpoint\":\"\",\"domain\":\"CUSTOMER_FINANCE\",\"component\":\"abctestProcessor\",\"log_type\":\"ERROR\",\"space_name\":\"development\",\"event_source\":\"DEEP_PROXY\",\"api_name\":\"test_abc\",\"api_id\":\"a36c4e54-dc5a-4d23-afb3-10f1661b19b4\",\"message_format\":\"application/json\",\"error_code\":0,\"stack_trace\":\"com.tmobile.deep.abc.exception.FinanceSystemE\"operation_name\":\"testEquipmentSerialNumberUpdateCompleted\",\"testId\":\"testString\",\"msisdn\":\"testString\",\"guid\":\"testString\",\"activityid\":\"testString\",\"api_request\":{\"eventId\":\"event123\",\"sourceId\":null,\"eventType\":\"testEquipmentSerialNumberUpdateCompleted\",\"eventTime\":{\"offset\":{\"totalSeconds\":0,\"id\":\"Z\",\"rules\":{\"fixedOffset\":true,\"transitions\":[],\"transitionRules\":[]}},\"hour\":0,\"minute\":30,\"second\":21,\"nano\":298000000,\"year\":2018,\"month\":\"FEBRUARY\",\"dayOfMonth\":10,\"dayOfWeek\":\"SATURDAY\",\"dayOfYear\":41,\"monthValue\":2},\"eventProducerId\":\"Produce123\",\"eventVersion\":\"testString\",\"specifications\":[{\"name\":\"testString\",\"value\":\"testString\"}],\"auditInfo\":{\"customerId\":\"testString\",\"accountNumber\":\"testString\",\"universalLineId\":\"testString\",\"lineId\":\"testString\",\"phoneNumber\":\"testString\",\"iamUniqueId\":\"testString\",\"batchId\":\"testString\",\"orderId\":\"testString\"},\"headerReference\":{\"activityId\":\"testString\",\"applicationId\":\"testString\",\"applicationUserId\":\"testString\",\"authCustomerId\":\"testString\",\"authFinancialAccountId\":\"testString\",\"authLineOfServiceId\":\"testString\",\"channelId\":\"testString\",\"dealerCode\":\"testString\",\"interactionId\":\"testString\",\"masterDealerCode\":\"testString\",\"segmentationId\":\"testString\",\"senderId\":\"testString\",\"sessionId\":\"testString\",\"storeId\":\"testString\",\"terminalId\":\"testString\",\"tillId\":\"testString\",\"workflowId\":\"testString\",\"timestamp\":{\"offset\":{\"totalSeconds\":0,\"id\":\"Z\",\"rules\":{\"fixedOffset\":true,\"transitions\":[],\"transitionRules\":[]}},\"hour\":0,\"minute\":30,\"second\":21,\"nano\":298000000,\"year\":2018,\"month\":\"FEBRUARY\",\"dayOfMonth\":10,\"dayOfWeek\":\"SATURDAY\",\"dayOfYear\":41,\"monthValue\":2}},\"payload\":{\"createtestRequest\":{\"header\":{\"senderid\":\"testString\",\"channelid\":\"testString\"},\"tests\":{\"account\":{\"universalLineId\":\"testString\"},\"sourceTransactionTime\":\"2018-02-10T00:30:21.298Z\",\"phoneNumber\":\"testString\",\"purchasedEquipment\":{\"description\":\"testString\",\"imei\":\"testString\"},\"testId\":\"testString\"}}},\"processContext\":{\"rootId\":\"67310650-1e3b-11e8-945d-a5cf584f50bc\",\"parentId\":\"67310650-1e3b-11e8-945d-a5cf584f50bc\",\"spaceName\":\"development\"},\"currentRetryCount\":0,\"maxRetryAttempts\":0,\"retryDelay\":0,\"taskId\":null,\"errorData\":null,\"status\":null,\"subStatus\":null},\"api_response\":\"org.hibernate.exception.GenericJDBCException: Error calling CallableStatement.getMoreResults\",\"httpStatusCode\":\"503\",\"key\":\"testString\",\"additionalAttributes\":{}}"
| search "\"eventid\":\"event123\""
| rex "{\"eventId\":\"(?<eventId>[^\"]+)"
| rex ",\"payload\":(?<payload>.*),\"api_response\""
| table eventId payload
PS: I have retained eventId field in case you need it to be displayed in result.
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