Write a program to determine whether user's input qualifies as a valid c++ identifier. Use a while loop to read a string that represents an identifer char by char. The program applies the actual rules for a valid c++ identifier and then let the user know the result. Examples are listed below.

Enter a valid c++ identifier.


this is not a valid identifier .

Enter a valid c++ identifier.


this is valid

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#include <cstdlib>
#include <iostream>
#include <string>
#include <stdlib.h>
using namespace std;

bool isValidIdentifier(string key){
    char c;
    if (key.length() <= 0) {
       return false;
    // An identifier is an arbitrarily long sequence of digits,
    // underscores, lowercase and uppercase Latin letters,
    // and most Unicode characters
    // but some compilers doesn't support UTF-8 very well
    for (int i = 0; i < key.length(); i++) {
       c = key[i];
       if (isalnum(c) == false
                && c != '_') {
            return false;

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