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