Untitled

%{
#include <stdio.h>
int state = 0; // Initial state
%}

%%
 
"a"    { 
           if (state == 0 || state == 1 || state == 3 || state == 4) state++;
           else state = 0;
       }
"b"    {
           if (state == 2) state = 0;
           else if (state == 0 || state == 1 || state == 3 || state == 4) state = 0;
       }
"ab"   {
           if (state == 2) state = 3;
           else if (state == 1) state = 2;
           else if (state == 3 || state == 4) state = 4;
       }
"abb"  {
           if (state == 3 || state == 4) state = 4;
           else state = 0;
       }
"abab" {
           if (state == 4) {
               printf("Input accepted\n");
               state = 0;
           } else {
               printf("Input rejected\n");
               state = 0;
           }
       }
\n     {
           state = 0;
       }
.      {
           printf("Input rejected\n");
           state = 0;
       }

%%

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