The former (%s) declares inclusive start

  the latter (%x) declares exclusive start
 



%x comment
%%
    int line_num = 1;
"/*" BEGIN(comment);
<comment>[^*\n] ;
<comment>"*"+[^*/\n]* ;/*Perhaps you could wonder if it contains the kind "***" or more "*".The myths is that it match one "*" every time*/
<comment>\n ++line_num;
<comment>"*/" {printf("%d lines\n",line_num); BEGIN(INITIAL); }
%%

int main()
{
    yylex();

    return 0;
}

int yywrap()
{
    return 1;
}