using System;
using System.Collections.Generic;
using System.Text.RegularExpressions;
using System.Linq;
public class Test
{
	public static void Main()
	{
		var userDic = new Dictionary<string, string> {
			{"399969178745962506", "One"},
			{"104729417217032192", "Two"}
		};
		var p =  @"<@!?(\d+)>";
		var s = "<@399969178745962506> hello to <@!104729417217032192>";
        Console.WriteLine(
        	Regex.Replace(s, p, m => userDic.ContainsKey(m.Groups[1].Value) ?
        		userDic[m.Groups[1].Value] : m.Value
        	)
        );
        Console.WriteLine(
        	Regex.Replace(s, @"(<@!?)(\d+)>", m => userDic.ContainsKey(m.Groups[2].Value) ?
        		$"{m.Groups[1].Value}{userDic[m.Groups[2].Value]}>" : m.Value
        	)
        );
	}
	
	
}