parser for expressions in lex/yacc - parsing

Question

I'm trying to parse statements to see whether they are valid or not according to these rules:
assignment:
id = exp ;
expression:
id op id {op id}
id is combination of digits and char, the first position contain a char.
I'm getting syntax error when I have something like this in my in.txt file: hellow = three3
but that shouldn't be a syntax error, and then when i put something like: hellow = =
that does not display a syntax error but it should. What am i doing wrong?
Lex:
%{
#include "y.tab.h"
#include <stdio.h>
%}
%%
[ \t\n]+ ;
[a-zA-Z][a-zA-Z0-9]* {
ECHO;
return ID;
}
%%
YACC:
%{
#include <stdio.h>
extern FILE * yyin;
%}
%token ID
%left '+' '-'
%left '*' '/' '%'
%right ';'
%%
assignment: expression
|
ID '=' expression
;
expression: ID
|
expression '*' expression
|
expression '/' expression
|
expression '%' expression
|
expression '+' expression
|
expression '-' expression
|
'(' expression ')'
;
%%
int main(void) {
yyin = fopen("in.txt", "r");
yyparse();
fclose(yyin);
return 0;
}

Answer 1

I can't say from just looking at this, but a good way to find out is running your parser when the environment variable YYDEBUG is 1, like in (unix):
YYDEBUG=1 ./a.out
You should get a detailed list of the steps the parser takes and the tokens it receives from the lexer (where the error is in most cases, like in your example: how does the parser ever get the '=' sign and the operators?). Compare this with the y.output file you get when you run yacc -v