using System;
using System.Text.RegularExpressions;
 
public class Test
{
	public static void Main()
	{
		string[] fail = {"ABCDacbd", "ACDE", "ABCDE\n", "_01234", "ABCDÉ", "ABCD́Ē", "ABCDEF_", "A_B_CDEF", "AB_C", "1234567890123456789012345", "123456_789012345678901234"};
		string[] ok = {"ACBDEF", "01234", "ABC_DE1", "123456789012345678901234", "12345_789012345678901234"};
 
		foreach (string s in fail) {
			Console.WriteLine(s + " " + Validate(s));
		}
		Console.WriteLine();
 
		foreach (string s in ok) {
			Console.WriteLine(s + " " + Validate(s));
		}
	}
 
	private static bool Validate(string str) {
		if (str.Length < 5 || str.Length > 24) {
			return false;
		}
 
		return Regex.IsMatch(str, @"^[A-Z0-9]+(?:_[A-Z0-9]+)?\z");
	}
}

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