^[0-9][0-9]?[^A-Za-z0-9]?po$
You can test it here: http://www.regextester.com/
To use this in C#,
Regex r = new Regex(@"^[0-9][0-9]?[^A-Za-z0-9]?po$");
if (r.Match(someText).Success) {
//Do Something
}
Remember, @ is a useful symbol that means the parser takes the string literally (eg, you don't need to write \\ for one backslash)