Flex Scanner continuously running after implementing steps from authoritative book

I'm on Linux Ubuntu.

I'm reimplementing the Flex scanner example provided at page 5 of "Managing projects with GNU make" by ROBERT MECKLENBURG.

count_words.c (application source code):

#include <stdio.h>
#include <stdlib.h> 

extern int fee_count, fie_count, foe_count, fum_count; 
extern int yylex( void ); 

int main( int argc, char ** argv )
{
 yylex();
 printf( "%d %d %d %d\n", fee_count, fie_count, foe_count, fum_count );
 exit( 0 );
}

lexer.l (input file of the Flex scanner):

        int fee_count = 0;
        int fie_count = 0;
        int foe_count = 0;
        int fum_count = 0;

%%
fee fee_count++;
fie fie_count++;
foe foe_count++;
fum fum_count++;

Makefile:

count_words: count_words.o lexer.o -lfl
 gcc count_words.o lexer.o -lfl -ocount_words
count_words.o: count_words.c
 gcc -c count_words.c
lexer.o: lexer.c
 gcc -c lexer.c
lexer.c: lexer.l
 flex -t lexer.l > lexer.c

Proper tabs are inserted before each command.

Not pasting lexer.c because it's huge but here's a link: https://drive.google.com/file/d/1lN7Jr93xa5hVKIHtCZgXmGRy1u3x4eE3/view?usp=sharing

When I run ./count_words This is a test fue , I get a blank output as if there was an infinite loop.

What is wrong?

I checked that Flex is installed. It is:

$which flex
/usr/bin/flex

I checked that libfl.a is installed. It is:

libfl-dev/jammy,now 2.6.4-8build2 amd64 [installed]
  static library for flex (a fast lexical analyzer generator)

I did notice that the book forgets to include stdlib.h for exit() and did include it.

I tried to find issues in lexer.c but the file is too complicated for me.

The book later recommends to append the following lines to lexer.l:

.
\n

I tried it but it does not solve the problem (no surprise).