docs: private static String parseNonDigits(final String expression, final int startIndex) {
          final StringBuilder operatorBuffer = new StringBuilder();
  
          char currentCharacter = expression.charAt(startIndex);
          int subExpressionIndex = startIndex;
          do {
              operatorBuffer.append(currentCharacter);
              subExpressionIndex++;
  
              if (subExpressionIndex < expression.length()) {
                  currentCharacter = expression.charAt(subExpressionIndex);
              }
          } while (subExpressionIndex < expression.length() && !Character.isDigit(currentCharacter) && !isSeparatorToken(currentCharacter));
  
          return operatorBuffer.toString();
      }