题解

#include<iostream>
#include<cstring>
using namespace std;
string s;
bool is_d(char c){
    return c>='0'&&c<='9';
}
bool is_le(char c){
    return (c>='a'&&c<='z')||(c>='A'&&c<='Z');
}
bool is_l(string s){
    if(s[0]>='0'&&s[0]<='9') return false;
    for(int i=0;i<s.size();i++){
        if(!is_d(s[i])&&!is_le(s[i])&&s[i]!='_')  return false;
    }
    return true;
}
int main(){
    cin>>s;
    if(is_l(s)) cout<<"yes";
    else cout<<"no";
}