Don't you mean "^[Cc][Tt][Nn]\d+$"? (Not written as a Java String literal.)
Java expression: String re = "^[Cc][Tt][Nn]\\d+$";
Is this homework?
Either way, you have to experiment with solutions you find here on Usenet. I
could be sadly mistaken, or Z could, or anyone could. OTOH, I did look up one
regex reference before posting this response.