docs: public static boolean isDigit(String str, int beginIndex, int endIndex)
      {
          char c;
          for ( int j = beginIndex; j < endIndex; j++ ) {
              c = str.charAt(j);
              //make full-width char half-width
              if ( c > 65280 ) c -= 65248;
              if ( c < 48 || c > 57 ) {
                  return false;
              }
          }
  
          return true;
      }