C++ :

#include <bits/stdc++.h>
using namespace std;

int main(){
	string s;
	//读入字符串 
	getline(cin,s);
	bool r = true;
	
	int i;
	//循环看有没有字母、数字、下划线以外的字符
	for(i = 0;i < s.size();i++){
		//含有非法字符 
		if((isalpha(s[i]) || isdigit(s[i]) || s[i] == '_') == false){
			r = false;
			break;
		}
	} 
	
	//判断不能以数字开头
	if(isdigit(s[0])){
		r = false;
	} 
	
	//判断不是四个特殊含义的单词
	if(s == "int" || s == "double" || s == "cout" || s == "cin") {
		r = false;
	}
	
	if(r == true){
		cout<<"yes"<<endl;
	}else{
		cout<<"no"<<endl;
	} 
	
	
    return 0;
}

Java :

import java.util.Scanner;

public class Main {
	public static void main(String args[]){
		Scanner sc = new Scanner(System.in);
		String str = sc.nextLine();
		if(str.contains(" ")||str.equals("int")||str.equals("double")||str.equals("cout")||str.equals("cin")||str.contains("#")||str.startsWith("0")||str.startsWith("1")||str.startsWith("2")||str.startsWith("3")||str.startsWith("4")||str.startsWith("5")||str.startsWith("6")||str.startsWith("7")||str.startsWith("8")||str.startsWith("9")){
			System.out.println("no");
		}else{
			System.out.println("yes");
		}
	}
}