5.4 Comments in the Input

Flex supports C-style comments, that is, anything between ‘/*’ and ‘*/’ is considered a comment in the parts of the file Flex interprets. 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:

%{
/* C code block - other target languages might have different comment syntax */
%}

/* Definitions Section */
%x STATE_X

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

If the target language is something other than C/C++, you will need to use its normal comment syntax in actions and code blocks. Note that the optional { and } delimiters around actions a Flex syntax, not C syntax; you will be able to use those even if, e,g., your target language is Pascal-like and delimits blocs with begin/end.