That regex doesn't make much sense 
If you want to look for
\"mysensitivefield\":\"ASDASDLAASDASDASLDASDAZ\"
and transform it to
\"****\":\"****\"
one way would be to capture the separators around the value and use them to create a new string with asterisks and the captured separators. There is a also some single/double-quote juggling so I'm not going to claim ownership of this configuration 
gsub => [ "message", '"mysensitivefield(\\\":\\\").*?(\\\")', '"****\1****\2' ]