I've been looking for ages to try and find an answer to this problem but haven't found an example of this being done to help me work out how to implement it. 

I'm creating a table and I want a column in MS SQL server which is CHAR(5) datatype but I want it to only allow first 2 CHARS of it to have a letter and following 3 CHARS to be numbers only. I've seen things like "CHECK (UnitCode NOT LIKE '%[^A-Z0-9]%') " which limits to only letters and numbers, but doesn't force to only allow which chars can or can't be numbers or letters.

IF someone can point me in the right direction I'd really appreciate it. Thanks.