[c#] Regex for 1 or 2 digits, optional non-alphanumeric, 2 known alphas

^[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)