Cat in Lex

cat.l

%%

    /* match everything except newline */
.   ECHO;

    /* match newline */
\n  ECHO;

%%

int yywrap(void) {
    return 1;
}

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

Makefile

cat: lex.yy.c
	cc -o cat lex.yy.c -ll

lex.yy.c: cat.l
	lex cat.l

clean:
	rm -f lex.yy.c
	rm -f cat