Sample lex+yacc program

addition.l

%{
#include <stdlib.h>
#include "y.tab.h"
%}
%%
[0-9]+ {
	yylval = atoi(yytext);
	return ENTIER;
}
[-+\n] return *yytext;
[ \t] ; /* skip whitespace */
. yyerror("invalid character");
%%

addition.yacc

%{
#include <stdio.h>
#include <string.h>

void yyerror(const char *str)
{
        fprintf(stderr,"Erreur: %s\n",str);
}
yywrap()
{
  return 1;
}
main()
{
 return yyparse();
}

%}

%start program
%token ENTIER
%left '+'

%%
program:
	program expr '\n' { printf("%d\n", $2); }
	| 
	;
expr:
	ENTIER { $$ = $1;}
	| expr '+' expr { $$ = $1 + $3; }
	;
%%