example.c

%{
#include <stdio.h>     /* printf() */
#include <stdlib.h>    /* atoi() */
int vars[26];
%}

Stmt   = - e:Expr EOL                  { printf("%d\n", e); }
       | ( !EOL . )* EOL               { printf("error\n"); }

Expr   = i:ID ASSIGN s:Sum            { printf("%s = %d\n", i, s); $$ = vars[i] = s; }
       | s:Sum                         { $$ = s; }

Sum     = l:Product
               ( PLUS  r:Product       { l += r; }
               | MINUS r:Product       { l -= r; }
               )*                      { $$ = l; }

Product = l:Value
               ( TIMES  r:Value        { l *= r; }
               | DIVIDE r:Value        { l /= r; }
               )*                      { $$ = l; }

Value   = i:NUMBER                      { $$ = atoi(yytext); }
       | i:ID !ASSIGN                  { $$ = vars[i]; }
       | OPEN i:Expr CLOSE             { $$ = i; }

NUMBER  = < [0-9]+ >    -               { $$ = atoi(yytext); }
ID      = < [a-z]  >    -               { $$ = yytext[0] - 'a'; }
ASSIGN  = '='           -
PLUS    = '+'           -
MINUS   = '-'           -
TIMES   = '*'           -
DIVIDE  = '/'           -
OPEN    = '('           -
CLOSE   = ')'           -

-       = [ \t]*
EOL     = '\n' | '\r\n' | '\r' | ';'

%%

int main()
{
 while (yyparse())
   ;
 return 0;
}