using System;

using System.Text.RegularExpressions;

​

public class Example

{

    public static void Main()

    {

        string pattern = @"&(?!(lt;|gt;))";

        string substitution = "ABC";

        string input = @"<ali>ulaşabilirsiniz\.&lt;br&gt;Kapatma &aliatay & test 01</ali>";

        RegexOptions options = RegexOptions.Multiline;

        Regex regex = new Regex(pattern, options);

        string result = regex.Replace(input, substitution);

        Console.WriteLine(input);

        Console.WriteLine(result);

    }

}

​