答疑

#include <iostream> 
using namespace std; 
int main() 
{ 
 string s; 
 cin>>s; 
 if(s[0]>='0'&&s[0]<='9'){ 
  cout<<"no"<<endl; 
  return 0; 
 } 
 for(int i=0;i<s.length();i++){ 
  if(!((s[i]>='0'&&s[i]<='9')||(s[i]>='A'&&s[i]<='Z')||(s[i]>='a'&&s[i]<='z')||s[i]=='_')) 
 { 
 cout<<"no"<<endl; 
 return 0; 
 } 
}   cout<<"yes"<<endl; 
return 0; 
}