Removing characters from a string via Terminal
This seems to work:

$ echo 'http://t.umblr.com/redirect?z=https%3A%2F%2Farchiveofourown.org%2Fworks%2F1885509&amp;t=N2M3YmNhNjQzODU5MjI2NmE5ZjljY2Y4NWVhOTVhM2IwYzgyYzYyYixLOHhQakVRbA%3D%3D' | awk 'BEGIN{FS="=|&amp;"}{gsub(/%3A/, ":")}{gsub(/%2F/, "/")}{print $2}'
https://archiveofourown.org/works/1885509
$


What happening here is the URL string is placed in single quotes and using echo is piped | into an awk program.

Example Syntax:

echo 'URL' | awk 'BEGIN{FS="=|&amp;"}{gsub(/%3A/, ":")}{gsub(/%2F/, "/")}{print $2}'