media\/([^\/?]*+) might work if you're only interested in that specific pattern. I was trying to test it but it seems to keep crashing my page.
For reference here's a good doc on regex in Dynatrace: https://www.dynatrace.com/support/help/organize-monitored-entities/reference/how-do-i-use-regular-expressions-in-dynatrace/
post processing options allow for splitting the values, which will create an array of the different parts (assuming the delimiter is /). The question is how do I retrieve only the second element of that array
the split actually puts multiple values on the request, so that is not what you want here.
instead of that remove the split ad do a regex that will go for the second part. this regex should do the trick, it jumps over part one and takes part two.