code: public static String isbnFormat(final String pstring) {
      if (pstring == null) {
        return null;
      }
      final int length = isbnCompress(pstring).length();
      if (length > Isbn10Validator.ISBN10_LENGTH) {
        return isbn13Format(pstring);
      } else {
        return isbn10Format(pstring);
      }
    }
docs: format isbn10 or 13 with separator signs.
  
  @param pstring string to format
  @return formated string