5.4 Comments in the Input

Flex supports C-style comments, that is, anything between ‘/*’ and ‘*/’ is considered a comment. Whenever flex encounters a comment, it copies the entire comment verbatim to the generated source code. Comments may appear just about anywhere, but with the following exceptions:

• Comments may not appear in the Rules Section wherever flex is expecting a regular expression. This means comments may not appear at the beginning of a line, or immediately following a list of scanner states.
• Comments may not appear on an ‘%option’ line in the Definitions Section.

If you want to follow a simple rule, then always begin a comment on a new line, with one or more whitespace characters before the initial ‘/*’). This rule will work anywhere in the input file.

All the comments in the following example are valid:

     %{
     /* code block */
     %}
     
     /* Definitions Section */
     %x STATE_X
     
     %%
         /* Rules Section */
     ruleA   /* after regex */ { /* code block */ } /* after code block */
             /* Rules Section (indented) */
     <STATE_X>{
     ruleC   ECHO;
     ruleD   ECHO;
     %{
     /* code block */
     %}
     }
     %%
     /* User Code Section */