Hi, I'm working on a akamai json and I want to extract the OS name from the message.UA field. Basically, if you look at the fake sample string below, I only want to get Windows (the part of the string between the first ( and the next %: Mozilla%2f5.0%20(Windows%20NT%2018.0%3b%20Win64%3b%20x64)%20AppleWebKit%2f580.36%20(KHTML,%20like%20Gecko)%20Chrome%2f81.0.4042.140%20Safari%2f537.36 I already created a regular expression that will do exactly what I want but I'm not able to make it work with rex (As you can imagine I'm new in Splunk). Here is how I'm trying to use it. index=akamai | regex field = message.UA "(?<=\()(.*?)(?=\%)"| top message.UA When I run it I get a Error in 'rex' command: The regex '(?<=()(?=\%)' does not extract anything. It should specify at least one named group. Format: (?...). Any idea of how to accomplish this extraction? Thanks! ... View more