@Jenny Franklin, I have also used ReplaceRegEx() just once, but in this case, it would seem you could do it as follows, @Doug Copeland:

You could do it with a single formula, but the set of two sounds cleaner:
CC Pointer (#) =Search("N3",WBS)β
CC (A) =If(CC Pointer>0,Mid(WBS,CC Pointer,4),"")
I generated random data, with an N thrown somewhere into the string, and the logic seems to hold. If it finds "N3" anywhere in WBS, it will take the next 2 characters.
OK, it doesn't check whether there are enough characters afterwards, but if you need that logic, I guess you can figure it out yourself, depending on your naming conventions ππ
I hope this helps.