Ok, So i would "think" that the intent of the regex is to look at the source of the data, and extract from that either a policy or email value (possibly both) however, the syntax for an inline rex expression is wrong!
I am assuming that the source of these logs is something like: e:\logs\policy\MyPolicy\somelog.log
(but that is quite a wild shot in the dark)
If I am correct (or close) you should update the query to: | rex field=source "Policy\\+(?<policy>.*)\\" | rex field=source "Email\\+(?<email>.*)\\"
Which basically says "look for a string beginning with the word "Policy" followed by a single "\" then extract all the characters before the trailing "\", and write them to a field called "policy"
If you share you source name, I can validate (or correct) my guess.