Compiler Design : Displaying the tokens in a statement entered by the user [C++]

Input:

The user will input strings like :

------------------
    int a = 12;
------------------
    float b = 23.32;
------------------
    a + b;
------------------
   a+12
------------------

Output:

 ***
---------------------
     Keyword: int
     Identifier: a
     Operator: =
     Constant: 12
---------------------

    Keyword: float
    Identifier b
    Operator: =
    Constant: 23.23
---------------------
    Identifier: a
    Operator: +
    Identifier: b
---------------------
[Error] : Statements must be terminated by a semicolon!
---------------------

 ***
This program will use the space to separate the various tokens and then identifies them. So for example, this won't work:

int a=12;

We were given the old Turbo C++ compiler for use in our college. So writing this was a lot of mess. -.-
Anyways, here is the code:

#include <iostream.h>
#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <conio.h>

char input[50];
char keyword[50][50] = { "int", "float", "double", "byte", "long" };
char op[50][50] = { "+", "-", "/", "*", "%", "=", "-=", "+=" };

void check(char *buff){
 //check for keywords:
 for (int i = 0; i < 5; ++i){
  if (strcmp(keyword[i], buff) == 0){
   cout << "\nKeyword: " << buff;
   return;
  }
 }

 //check for operators:
 for (int j = 0; j < 8; ++j){
  if (strcmp(op[j], buff) == 0){
   cout << "\nOperator: " << buff;
   return;
  }
 }

 if (buff[0] == '!'){
  cout << "\nOperator: " << '!';
  cout << "\nIdentifier: ";
  int index = 1;
  while (buff[index] != '\0'){
   putchar(buff[index]);
   index++;
  }
  return;
 }

 //check for constants and identifiers:
 int index = 0;
 int isNotConstant = 0;
 while (buff[index] != '\0'){
  if (buff[index] == '.')
   ;
  else if (!isdigit(buff[index])){
   isNotConstant = 1;
   break;
  }
  index++;
 }

 if (isNotConstant){
  cout << "\nIdentifier: " << buff;
 }
 else
  cout << "\nConstant: " << buff;
}

int main(){
 clrscr();
 cout << "Enter the expression: ";
 gets(input);

 int last = -1;
 char buff[50];
 int error = 1;
 int buffIndex = 0;
 for (int i = 0; i < 50; ++i){
  if (input[i] == ';'){
   error = 0;
   for (int j = i; j >= 0; --j){
    if (last == -1)last = i;
    if (j == 0){
     for (int k = 0; k < last; ++k){
      buff[buffIndex++] = input[k];
     }
     buff[buffIndex++] = '\0';
     check(buff);
     buffIndex = 0;
    }
    if (input[j] == ' '){
     for (int k = j + 1; k < last; ++k){
      buff[buffIndex++] = input[k];
     }
     buff[buffIndex++] = '\0';
     check(buff);
     buffIndex = 0;
     last = j;
    }
   }
  }
 }
 if (error)
  cout << "\n[Error] : Statements must be terminated by a semicolon!";
 cout << endl;
 getch();
 return 0;
}