match_c_comments.lex

%x IN_COMMENT

%%

<INITIAL>{
     "/*"      BEGIN(IN_COMMENT);
}
<IN_COMMENT>{
     "*/"      BEGIN(INITIAL);
     [^*\n]+   // eat comment in chunks
     "*"       // eat the lone star
     \n        yylineno++;
}

%%

main() {
   yylex();
   return 0;
}

input.txt

#include<stdio.h>

main() {
   /*
     comentariu
   */
   printf("*/");

   return 0;
}