code: public static RegExpr createKeyword(String str) {
          int i;
          RegExpr[] chars;
  
          if (str.length() == 1) {
              return new Range(str.charAt(0));
          } else {
              chars = new RegExpr[str.length()];
              for (i = 0; i < chars.length; i++) {
                  chars[i] = new Range(str.charAt(i));
              }
              return new Sequence(chars);
          }
      }
docs: returns a Range for strings of length 1