How To Add New Staff Number?

i have a field called staff_number contains staff number like this:

DR1, DR20, SR57, DR165, etc.. where DR is a code for Doctors
NU17, NU564, NU9, etc... where NU is a code for Nurse

i am passing the code (DR, NU, etc) from my C# application but i want the SQL to add one more number to the last number..

so it should check if I am passing DR as code for the last DR, let's say DR14 so the next DR will be DR15


i don't know how to this because the number has no fix length so it's from 1 to 999