I duplicate your example and then changed the IP's and customer names. The below regex works unless your events change format. I ran a test with your example data as sometimes the rex does not work the same as regex101.com
You highlighted "abc" as the customer name. The Splunk auto extract of the field=value pair does not properly work with your data as demonstrated with the below search
index=main | rex "START\:\s(?((\d{1,3}\.){3}\d{1,3}))\s.*?customer_name\=(?[a-zA-Z\d\s:]+)" | table _time customer_name cust_name ipaddr
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