please help on infix yylex.

[Laxman Bana]

Lex Gurus,
  I am trying to learn flex and bison and I started with infix calc example 
specified in the manual.
Everything is nomal when I use yylex() function specified in the manual. But 
I wanted to generate tokens using flex and I created following rules and it 
never worked....:(
I tried to fix this for long time but I am not able to see where I am making 
a mistake? Please help.
Thanks,
Laxman

/* infix.l */
%{
#include <math.h>
#include <infix.tab.h>
extern int yylval;
%}
WS      [ \t\n]
DIGIT   [0-9]
%%
{DIGIT}+        { yylval = atoi(yytext); return NUM; }
{WS}    ; /* eat empty spaces. */
.       return yytext[0];
%%

yywrap()
{
  return 1;
}

/* infix.y */
%{
#define YYSTYPE int
#include <math.h>
#include <stdio.h>
%}
%token NUM
%%
input:
        | input line
        ;
line:   '\n'
        |       exp '\n'        { printf ("\t= %d\n", $1); }
        ;
exp:    NUM                     { $$ = $1; }
        | exp '+' exp   { $$ = $1 + $3; }
        | exp '-' exp   { $$ = $1 - $3; }
        | exp '*' exp   { $$ = $1 * $3; }
        | exp '/' exp   { $$ = $1 / $3; }
        ;
%%

main()
{
   yyparse();
}
yyerror(s)
   char *s;
{
   printf("%s \n", s);
}

commands:
$ flex -oinfix.l.c infix.l ; bison -d -oinfix.tab.c infix.y
$ gcc -I.  -o infix  infix.l.c infix.tab.c
flex version : 2.5.4
Bison version: 1.875b