import java.util.*;
import java.util.regex.*;

class Ideone
{
	public static String username(String str) {
	    return Boolean.toString( str.trim().matches("[a-zA-Z]\\w{3,24}$(?<!_)") );
	}
	public static void main (String[] args) throws java.lang.Exception
	{
		System.out.println(username("u__hello_world123"));
		System.out.println(username("aa_"));
	}
}