***** lex part *****

%{
#include <stdlib.h>
#define NUM 257
double yylval;
%}
INT     [0-9]+
NUM     ([+-]?)(\.{INT}|{INT}(\.{INT}?)?)
%%
{NUM}   {yylval = atof(yytext);  return NUM;}
[ \t]*  ;
.|\n    {return yytext[0];}
%%
int yywrap()  {return 1;}

***** yacc part *****

%{
#include <stdio.h>
void yyerror(char*s)  {printf("%s\n",s);}
extern int yylex();
extern int yywrap();
#define YYSTYPE double
%}
%token NUM
%%
input: /**/ | input line
;
line:   '\n' | rpn '\n' {printf("%f\n",$1);}
      | error '\n' {yyerrok;}
;
rpn:    NUM
      | rpn rpn '+'  {$$=$1+$2;}
      | rpn rpn '-'  {$$=$1-$2;}
      | rpn rpn '*'  {$$=$1*$2;}
      | rpn rpn '/'  {$$=$1/$2;}
;
%%
int main()  {yyparse(); return 0;}