code: private String getSugarSymbol() {
      String sugarSymbol = null;
      // nucleotide notation
      String ncNotation = getNotation();
      // nucleotide has base
      if (ncNotation.contains("(")) {
        int start = ncNotation.indexOf("(");
        sugarSymbol = ncNotation.substring(0, start);
        sugarSymbol = sugarSymbol.replaceAll("\\[|\\]", "");
      } else {
        if (getPositionType() == STARTING_POSITION_TYPE) {
          if (ncNotation.endsWith("]")) {
            char[] chars = ncNotation.toCharArray();
            for (int i = chars.length; i > 0; i--) {
              String letter = String.valueOf(chars[i - 1]);
              if (letter.equals("[")) {
                sugarSymbol = ncNotation.substring(0, i);
                sugarSymbol = sugarSymbol.replaceAll("\\[|\\]", "");
                break;
              }
            }
          } else {
            sugarSymbol = ncNotation.substring(0, ncNotation.length() - 1);
            sugarSymbol = sugarSymbol.replaceAll("\\[|\\]", "");
          }
        } else {
          if (ncNotation.startsWith("[")) {
            sugarSymbol = ncNotation.substring(0, ncNotation.indexOf("]"));
            sugarSymbol = sugarSymbol.replaceAll("\\[|\\]", "");
          } else {
            sugarSymbol = ncNotation.substring(0, 1);
          }
        }
      }
      return sugarSymbol;
    }
docs: find the sugar symbol, if it is a modified monomer, the "[" and "]" will be
  removed
  
  @return sugar monomer ID