How to use the string variables in regex?

class Program
{
    static void Main(string[] args)
    {
        string s1 = "one_two";
        string s2 = "one_two_three";
        string s3 = "one_two_three_four";
        string s4 = "one]two[three-four";

        [UPDATE]
        Regex regex = new Regex(@"[_\[\]]");
        Regex regex = new Regex(@"[\S\s]");

        string result = "";
        result = regex.Replace(s1, new MatchEvaluator(Program.ReplaceWithHyphen));
        result = regex.Replace(s2, new MatchEvaluator(Program.ReplaceWithHyphen));
        result = regex.Replace(s3, new MatchEvaluator(Program.ReplaceWithHyphen));
        result = regex.Replace(s4, new MatchEvaluator(Program.ReplaceWithHyphen));

    }

    [UPDATE]
    static string ReplaceWithHyphen(Match m)
    {
        // Return the hyphen as a replacement if the character is not a letter or digit
        char ch = m.Value[0];
        if (Char.IsLetterOrDigit(ch))
            return ch.ToString();
        else
            return "-";
    }
}

List<string> words = new List<string>{"one_two",
            "one_two_three",
            "one_two_three_four",
            "one]two[three-four"};


var qry = words.Select(w=>w = new Regex(@"(\])|(\[)|(_)").Replace(w,"-"));

one-two
one-two-three
one-two-three-four
one-two-three-four

static void Main(string[] args)
{
    string[] searchElemnts = new string[] { "öne", "two","three", "four" };
    string[] samples = new string[] { "öne_two", "öne_two_three","öne_two_three_four", "öne]two[three-four" };

    string[] copySamples = samples.ToArray();

    for (int i = 0; i < copySamples.Length; i++)
    {
        foreach (string element in searchElemnts)
        {
            copySamples[i] = copySamples[i].Replace(element, "");
        }
    }

    char[] unwantedChars = string.Join("",copySamples.ToArray()).ToCharArray().Distinct().ToArray();
    Regex regex = new Regex("[" + Regex.Escape(new string(unwantedChars)).Replace("]", "\\]") + "]");

    string result = "";
    for (int i = 0; i < samples.Length; i++)
    {
        result = regex.Replace(samples[i], "-");
        System.Diagnostics.Debug.Print(result);
    }
}