12.5 Invalid Numbers

Some number-like constructs which are not really valid as numeric constants are treated as numbers in preprocessing directives. If these constructs appear outside of preprocessing, they are erroneous. See Preprocessing Tokens.

Sometimes we need to insert spaces to separate tokens so that they won’t be combined into a single number-like construct. For example, 0xE+12 is a preprocessing number that is not a valid numeric constant, so it is a syntax error. If what we want is the three tokens 0xE + 12, we have to use those spaces as separators.