If you need to get the last few characters of a string, you can just use awk.  This works great because you don’t have to know how long or short the string is to begin with. It first gets the length of the string. So in this example the length is 9 characters, and the ‘1’ would be in position 9 of the string. Then it subtracts 2 from position 9, to get position 7 (which is the ‘3’), then it gets 3 characters starting from position 7: ‘371’

# echo server371 | awk '{print substr($0,length($0)-2,3)}'
371

# echo anotherserver892 | awk '{print substr($0,length($0)-2,3)}'
892