Recognize aa(a+b)*bb

%{
#include <stdio.h>
int count=0;
%}
str     "aa"[ab]*"bb"
err     [a-zA-Z0-9]+
%%
{str} {    count++; }
{err}
%%
int main()
{
       yylex();
        printf("The pattern aa(a+b)*bb found %d times \n",count);
}
int yywrap()
{
        return 1;
}