function str_rot13 (str) {
  // http://kevin.vanzonneveld.net
  // +   original by: Jonas Raoni Soares Silva (http://www.jsfromhell.com)
  // +   improved by: Ates Goral (http://magnetiq.com)
  // +   bugfixed by: Onno Marsman
  // +   improved by: Rafał Kukawski (http://blog.kukawski.pl)
  // *     example 1: str_rot13('Kevin van Zonneveld');
  // *     returns 1: 'Xriva ina Mbaariryq'
  // *     example 2: str_rot13('Xriva ina Mbaariryq');
  // *     returns 2: 'Kevin van Zonneveld'
  // *     example 3: str_rot13(33);
  // *     returns 3: '33'
  return (str + '').replace(/[a-z]/gi, function (s) {
    return String.fromCharCode(s.charCodeAt(0) + (s.toLowerCase() < 'n' ? 13 : -13));
  });
}

str_rot13('Kevin van Zonneveld');

'Xriva ina Mbaariryq'

str_rot13('Xriva ina Mbaariryq');

'Kevin van Zonneveld'

str_rot13(33);

'33'