Problem

You have a file with lines like:

a::key1::b
c::d::key2

and you want to generate a csv that looks like:

a::key::b,key1
c::d::key2,key2

To extract the key you can use the sed command:

sed -E  's/.*(key[0-9]+).*/\1/g'

To extract the key and keep the original string:

ed -E  's/(.*(key[0-9]+).*)/\1,\2/g'