title: Invalid characters in C preprocessors/macros?
body: <p>What are the different <strong>invalid characters</strong> that I am not allowed use in a <strong>macro</strong> ?<br>
  It seems that <code>#define TE$T 8</code> is working, so <code>$</code> is valid.<br>
  Does somebody have a list of the invalid characters ? (or on the contrary the list of the valid ones).  </p>