debugging web application firewall errors

 The goal is to understand what is causing the request to fail on azure WAF. Example log :

AzureDiagnostics

| where ResourceProvider == "MICROSOFT.NETWORK"

| where Category == "ApplicationGatewayFirewallLog"

| where action_s == "Matched"

| project details_message_s, details_data_s

This will give and a 

details_message_s: Pattern match (?:\$(?:\((?:\(.*\)|.*)\)|\{.*\})|[<>]\(.*\)) at ARGS.

and some details_data_s

Copy those values to https://regex101.com/ to find where is fails.

https://techcommunity.microsoft.com/blog/azurenetworksecurityblog/azure-waf-tuning-for-web-applications/3776133