using System;
using System.Text.RegularExpressions;

using System;
using System.Text.RegularExpressions;

public class Test
{
  public static void Main()
  {
    var text = "123456789";
    var res = Regex.Replace(text, @"^((?<ch>.)*).((?<-ch>.)*(?(ch)(?!))$)", @"$1$2");

    Console.WriteLine(text);
    Console.WriteLine(res);
  }
}

string input = "aaa bbbb ccccc dddddd";
string pattern = @"w+";
string RemoveCentralSymbol(string s) => s.Remove(s.Length / 2, s.Length % 2);
var result = Regex.Replace(input, pattern, match => RemoveCentralSymbol(match.Value));
Console.WriteLine(result);