Add separator to string at every N characters?

There's another Regex approach:

var str = "111111110000000011111111000000001111111100000000";
# for .NET 4
var res = String.Join(",",Regex.Matches(str, @"\d{8}").Cast<Match>());

# for .NET 3.5
var res = String.Join(",", Regex.Matches(str, @"\d{8}")
            .OfType<Match>()
            .Select(m => m.Value).ToArray());

...or old school:

public static List<string> splitter(string in, out string csv)
{
     if (in.length % 8 != 0) throw new ArgumentException("in");
     var lst = new List<string>(in/8);

     for (int i=0; i < in.length / 8; i++) lst.Add(in.Substring(i*8,8));

     csv = string.Join(",", lst); //This we want in input order (I believe)
     lst.Reverse(); //As we want list in reverse order (I believe)

     return lst;
}

Regex.Replace(myString, ".{8}", "$0,");

If you want an array of eight-character strings, then the following is probably easier:

Regex.Split(myString, "(?<=^(.{8})+)");

which will split the string only at points where a multiple of eight characters precede it.

Try this:

var s = "111111110000000011111111000000001111111100000000";
var list = Enumerable
    .Range(0, s.Length/8)
    .Select(i => s.Substring(i*8, 8));
var res = string.Join(",", list);