You can try it with
(\d+).*/.*\d+
Explanation:
"Any character" can also be "no character".
Therefore it matches the number 1 from
1/2
as well as in
1 / 2
Please be aware that you can not undo this step.
The information about how many discs your current album has (2 in the above example) is gone after appying this regular expression. It only remains the discnumber itself.