C#

//Disclaimer Psuedocode & Untested!!!!
public string Foo(string original)
{
    string output = original;
    string remainder = outputstring.RemoveLastFourCharacters();
    while (remainder.Length > 0)
    {  
        output = remainder + "|" + output;
        remainder = remainder.RemoveLastFourCharacters();
    }
    return output;
}

string source = "/100/101/201/";
string resultWanted = "/100/|/100/101/|/100/101/201/";
string[] items = source.Split(new char[] { '/' }, StringSplitOptions.RemoveEmptyEntries);
StringBuilder item = new StringBuilder("/");
StringBuilder sb = new StringBuilder();
string separator = "";
foreach (string s in items)
    {
    item.Append(s);
    item.Append("/");
    sb.Append(separator + item);
    separator = "|";
    }
if (sb.ToString() == resultWanted)
    {
    MessageBox.Show("OK");
    }

Mycroft Holmes wrote:

I may even learn something from it.

string actual = "/100/101/201/";
var result = actual.ToCharArray()
                   .Select((x, y) => new { ch = x, pos = y })
                   .Where(x => x.ch == '/' && x.pos > 0)
                   .Select(x => actual.Substring(0, x.pos + 1))
                   .ToArray();
string derived = String.Join("|", result); 
Console.WriteLine(derived); 

Mycroft Holmes wrote:

the winner

Eddy Vluggen wrote:

Cute == obfuscated?

Eddy Vluggen wrote:

One method to divide them, one to combine them again