code: private void checkForEmptyIdentifier() {
      Character la = chars.lookahead(1);
      if (CharType.DOT.isMatchedBy(la) || CharType.PLUS.isMatchedBy(la)
          || CharType.EOL.isMatchedBy(la)) {
        throw new ParseException("Identifiers MUST NOT be empty", new UnexpectedCharacterException(la,
            chars.currentOffset(), CharType.DIGIT, CharType.LETTER, CharType.HYPHEN));
      }
    }
docs: Checks for empty identifiers in the pre-release version or build metadata.
  
  @throws ParseException
  if the pre-release version or build metadata have empty identifier(s)