Using REGEX_LIKE to Implement a Check Constraint
This isn't really a database administration question, but...

Firstly, your pattern does not prevent entries like this: XXXINT123abogus, because it does not say that "INT" or "NINT" should be at the beginning of the string, neither does it say that the lowercase letter, if it's there, should be the last symbol of the string.

You may want to try something like:

 +----------------------------- begins with
 |+---------------------------- optional "N"
 ||  +------------------------- then "INT"
 ||  |  +---------------------- then digits
 ||  |  | +-------------------- minimum 2
 ||  |  | | +------------------ maximum 4
 ||  |  | | |   +-------------- then a lowercase letter
 ||  |  | | |   |  +----------- which is optional
 ||  |  | | |   |  |+---------- and nothing more
 ||  |  | | |   |  ||
 ||  |  | | |   |  ||
'^N?INT\d{2,4}[a-z]?$'