nyoj-816-它合法吗？

#include<stdio.h>
int main()
{
    char a[12];
    while(scanf("%s",a)!=EOF)
    {
        int i;
        for(i=0;a[i]!='\0';i++)
        if(a[0]>='0'&&a[0]<='9') break;
        else if(a[i]=='_'||a[i]>='a'&&a[i]<='z'||a[i]>='A'&&a[i]<='Z'||a[i]>='0'&&a[i]<='9')
         continue;
         else break;
         if(a[i]=='\0') printf("Yes\n");
         else printf("No\n");
    }
    return 0;
}