docs: private char isQuote(int index) {
      if(index >= input.length()) {
        return 0;
      }
      char c = input.charAt(index);
      for(int i = 0; i < quoteChars.length; i++) {
        if(c == quoteChars[i]) {
          return c;
        }
      }
      return 0;
    }