simple flex example

count_words.c

#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

    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 -o count_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