#include<bits/stdc++.h>
using namespace std;
string n;
bool f=true;
int main(){
	getline(cin,n);
	for(int i=0;n[i];i++){
		if(i==0&&(n[i]>='0'&&n[i]<='9')){
			f=false;
			break;
		}
		if(!((n[i]>='a'&&n[i]<='z')||(n[i]>='A'&&n[i]<='Z')||(n[i]>='0'&&n[i]<='9')||n[i]=='_')){
			f=false;
			break;
		}
	}
	if(f) cout<<"yes";
	else cout<<"no";
	return 0;
}

这判断是真长啊

#include <bits/stdc++.h>
using namespace std;
int main(int argc, char **argv){
	char c;bool f = 0,idx = 0;
	while(cin >> c){
		if ((!idx) && c >= '0' && c <= '9'){
			f = 1;
			break;
		}
		if (!((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || (c >= '0' && c <= '9') || c == '_')){
			f = 1;
			break;
		}
		idx = 1;
	}
	if(f){
		cout << "no";
	}else{
		cout << "yes";
	}
	return 0;
}

#include<bits/stdc++.h>
using namespace std;
int main(){
	string a;
	cin>>a;
	
	if(!(a[0]>='a' && a[0]<='z' || a[0]>='A' && a[0]<='Z' || a[0]=='_')){ //开头仅为字母或下划线 
		cout<<"no";
		return 0; 
	}
	
	for(int i=1;i<a.size();i++){
		if(!(a[i]>='a' && a[i]<='z' || a[i]>='A' && a[i]<='Z' || a[i]=='_' || a[i]>='0' && a[i]<='9')){ //只包含字母、数字及下划线
			cout<<"no";
			return 0; 
		}
	}
	
	cout<<"yes";
	return 0;
}

#include<bits/stdc++.h>
using namespace std;
string a;
int main(){
   cin>>a;
   short len=a.length();
   for(int i=0;i<len;i++){
      if(a[0]>='0' and a[0]<='9'){
      cout<<"no";return 0;}
      if(!((a[i]>='A' and a[i]<='Z') or (a[i]>='a' and a[i]<='z') or (a[i]>='0' and a[i]<='9') or (a[i]=='_'))){
         cout<<"no";return 0;}
   }
   cout<<"yes";
   return 0;
}//不难就是长

#include<bits/stdc++.h>
using namespace std;
string n;
bool f=true;
int main(){
	getline(cin,n);
	for(int i=0;n[i];i++){
		if(i==0&&(n[i]>='0'&&n[i]<='9')){
			f=false;
			break;
		}
		if(!((n[i]>='a'&&n[i]<='z')||(n[i]>='A'&&n[i]<='Z')||(n[i]>='0'&&n[i]<='9')||n[i]=='_')){
			f=false;
			break;
		}
	}
	if(f) cout<<"yes";
	else cout<<"no";
	return 0;
}

除了判断没什么难的

using namespace std;
int maxx = 0x7fffffff;
int main()
{
	string a;
	bool flag=true;
	getline (cin,a);
	if ((!((65<=int(a[0]) && int(a[0])<=90) || (97<=int(a[0]) && int(a[0])<=122) || int(a[0])==95))){
		cout << "no";
		return 0;
	}
	for (int i=0;i<a.length();i++)
	{
		if (!((65<=int(a[i]) && int(a[i])<=90) || (97<=int(a[i]) && int(a[i])<=122) || (48<=int(a[i]) && int(a[i])<=57) || int(a[i])==95)){
			flag=false;
		}
	}
	if (flag){
		cout << "yes"; 
	}else{
		cout << "no";
	}
 }

#include<bits/stdc++.h> using namespace std; char a[100009]={0}; int main(){ gets(a); int s=strlen(a); if(a[0]>='0'&&a[0]<='9'){ cout<<"no"; return 0; } for(int i=0;i<s;i++){ if((a[i]!='_')&&(a[i]<'0'||(a[i]>'Z'&&a[i]<'a')||a[i]>'z')){ cout<<"no"; return 0; } } cout<<"yes"; }

#include<bits/stdc++.h> using namespace std; int correct[260]; int main(){ string s; getline(cin,s); int ssize=int(s.size()); for(int i=0;i<ssize;i++){ if(i==0&&s[i]>='0'&&s[i]<='9'){ cout<<"no"; correct[i]=1; correct[ssize-1]=1; break; } else if((s[i]<'0'||s[i]>'9')&&(s[i]<'a'||s[i]>'z')&&(s[i]<'A'||s[i]>'Z')&&s[i]!='_'){ cout<<"no"; correct[i]=1; correct[ssize-1]=1; break; } else{ correct[i]=0; } } for(int j=0;j<ssize;j++){

if(j==ssize-1&&correct[j]==0){
		cout<<"yes";
		break;
	}
	else{
	
	}
}
return 0;

}