I must use regex in order to get the last two digits of a year but only when 4 digits exist. I have the following regex which works perfectly when there is 4 digits. Example 20
The regex you have there shouldn't be working with 4 digits either. Your regex is looking for any 2 characters at the beginning of the string.
Try this:
(?<=\d\d)\d\d$
Debuggex Demo
This is different from Fede's answer in that you don't need to use and subsequently refer to a capturing group later. Only the last 2 digits are part of the match. It relies on a positive lookbehind.