How do I solve the symbols code coach ?

Question

Here's my code :
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Text.RegularExpressions;

namespace SoloLearn
{
class Program
{
static void Main(string[] args)
{
string input = Console.ReadLine();
string result = Regex.Replace(input, "[^a-zA-Z0-9_]+"," ");
Console.Write(result.Replace(" ",""));
}
}
}