Premature EOF during compilation of flex program

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Flex

Hello CPs,

I am new to Flex and I'm trying to understand how compilers and lexical analyzers work. I acquired this sample program from the internet and it doesn't seem to work. I am using Flex 2.5.4a. There were a few fixes but it didn't work. Are there any other syntactical errors or is the updated version changed any semantics. Given below is the program. While in the cmd-line when given the command flex -filename it returns the error message Premature end of file at line 32. Thanks in advance.

%{

  #include <stdlib.h>
  #include "y.tab.h"

%}

%%

("hi"|"oi")"\n"      {return HI; }
("tchau"|"bye")"\n"  {return BYE;}
.                    {yyerror(); }

%%

int main(void)
{
    yyparse();
    return 0;
}

int yywrap(void)
{
    return 0;
}

int yyerror(void)
{
    printf("Error\n");
    exit(1);
}