Parsing an Expression in the form <id> = <expr> - parsing

Question

When the program runs it asks me to write an expression to be parsed but when I type anything, I get this:
Enter an expression to be parsed below:
read * 3
Call lex...read
Enter <program>
Enter <statement>
Call lex...
ERROR: Must be of the form <id> = <expr>.
Here is the program:
#include <iostream>
#include <stdlib.h>
#include <cctype>
using namespace std;
int charClass;
char lexeme[100];
char nextChar;
int lexemeLength;
int token;
int nextmaiToken;
int i = 0;
string input;
int lex ();
void program ();
void statement ();
void expr ();
void term ();
void factor ();
#define LETTER 0
#define NUMBER 1
#define UNKNOWN 99
#define INT_CONSTANT 10
#define ID 11
#define ASSIGNMENT_OP 20
#define ADD_OP 21
#define SUB_OP 22
#define MULT_OP 23
#define DIV_OP 24
#define LEFT_PAREN 25
#define RIGHT_PAREN 26
#define EQUALS 27
#define END -1
void
addChar ()
{
if (lexemeLength <= 98)
{
lexeme[lexemeLength++] = nextChar;
lexeme[lexemeLength] = 0;
}
else
{
cout << "Error - lexeme is too long!\n\n";
exit (1);
}
}
void
getChar ()
{
nextChar = input[i++];
if (nextChar != END)
{
if (isalpha (nextChar))
charClass = LETTER;
else if (isdigit (nextChar))
charClass = NUMBER;
else
charClass = UNKNOWN;
}
else
charClass = END;
}
void
getNonBlank ()
{
if (isspace (nextChar))
getChar ();
}
int lookup (char ch)
{
switch (ch)
{
case '(':
addChar ();
nextmaiToken = LEFT_PAREN;
break;
case ')':
addChar ();
nextmaiToken = RIGHT_PAREN;
break;
case '+':
addChar ();
nextmaiToken = ADD_OP;
break;
case '-':
addChar ();
nextmaiToken = SUB_OP;
break;
case '*':
addChar ();
nextmaiToken = MULT_OP;
break;
case '/':
addChar ();
nextmaiToken = DIV_OP;
break;
case ';':
addChar ();
nextmaiToken = END;
break;
case '=':
addChar ();
nextmaiToken = EQUALS;
break;
default:
addChar ();
break;
}
return nextmaiToken;
}
int lex ()
{
lexemeLength = 0;
switch (charClass)
{
case LETTER:
addChar ();
getChar ();
while (charClass == LETTER || charClass == NUMBER)
{
addChar ();
getChar ();
}
nextmaiToken = ID;
break;
case NUMBER:
addChar ();
getChar ();
while (charClass == NUMBER)
{
addChar ();
getChar ();
}
nextmaiToken = INT_CONSTANT;
break;
case UNKNOWN:
lookup (nextChar);
getChar ();
break;
default:
nextmaiToken = END;
lexeme[0] = 'E';
lexeme[1] = 'O';
lexeme[2] = 'F';
break;
}
cout << "Call lex..." << lexeme << "\n";
return nextmaiToken;
}
void program ()
{
cout << "Enter <program>\n";
statement ();
if (nextmaiToken != END)
{
cout << "ERROR: No ending ';'\n\n";
exit (1);
}
else
{
cout << "Exit <program>\n";
cout << "This is a valid program.\n\n";
}
}
void statement ()
{
cout << "Enter <statement>\n";
if (nextmaiToken == ID)
{
lex ();
if (nextmaiToken == EQUALS)
{
lex ();
expr ();
}
else
{
cout << "ERROR: Must be of the form <id> = <expr>.\n\n";
exit (1);
}
}
else
{
cout << "ERROR: Must be of the form <id> = <expr>.\n\n";
exit (1);
}
cout << "Exit <statement>\n";
}
void expr ()
{
cout << "Enter <expr>\n";
term ();
while (nextmaiToken == ADD_OP || nextmaiToken == SUB_OP)
{
lex ();
term ();
}
cout << "Exit <expr>\n";
}
void term ()
{
cout << "Enter <term>\n";
factor ();
while (nextmaiToken == MULT_OP || nextmaiToken == DIV_OP)
{
lex ();
factor ();
}
cout << "Exit <term>\n";
}
void factor ()
{
cout << "Enter <factor>\n";
if (nextmaiToken == ID || nextmaiToken == INT_CONSTANT) //for the cases: <factor> -> id and <factor> -> int_constant
lex ();
else
{
if (nextmaiToken == LEFT_PAREN) //for the case: <factor> -> ( <expr> )
{
lex ();
expr ();
if (nextmaiToken == RIGHT_PAREN) //must close parentheses
lex ();
else
{
cout << "ERROR: No closing parentheses.\n\n";
exit (1);
}
}
else
{
cout << "ERROR: Unbalanced expression.\n\n";
exit (1);
}
}
cout << "Exit <factor>\n";
}
int main ()
{
cout << "\nEnter an expression:\n";
cin >> input;
cout << "\n";
nextChar = input[i];
getChar ();
lex ();
program ();
return (1);
}
The possible set of tokens, represented with regular expressions, includes:
= + - * / ( )
ID -> letter(letter|digit)*
Except read, write, and halt.
LETTER ->[a-zA-Z]
DIGIT ->[0-9]
What is missing or what is making stop early!!