Removing nested comments bz lex

Question

How should I do program in lex (or flex) for removing nested comments from text and print just the text which is not in comments? I should probably somehow recognize states when I am in comment and number of starting "tags" of block comment.

Lets have rules:
1.block comment

/*
block comment
*/

2. line comment

// line comment

3. Comments can be nested.

Example 1

show /* comment /* comment */ comment */ show

output:

show  show

Example 2

show /* // comment
comment
*/
show

output:

show 
show 

Example 3

show
///* comment
comment
// /*
comment
//*/ comment
//
comment */
show

output:

show
show

Answer 1

You got the theory right. Here's a simple implementation; could be improved.

%x COMMENT
%%
%{
   int comment_nesting = 0;
%}

"/*"            BEGIN(COMMENT); ++comment_nesting;
"//".*          /* // comments to end of line */

<COMMENT>[^*/]* /* Eat non-comment delimiters */
<COMMENT>"/*"   ++comment_nesting;
<COMMENT>"*/"   if (--comment_nesting == 0) BEGIN(INITIAL);
<COMMENT>[*/]   /* Eat a / or * if it doesn't match comment sequence */

  /* Could have been .|\n ECHO, but this is more efficient. */
([^/]*([/][^/*])*)* ECHO;  
%%

Answer 2

This is exactly what you need : yy_push_state(COMMENT) Its uses a stack to store our states which comes handy in nested situations.