Parsing text in C#.NET (a little problem)

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.NET

C#3.5

string input = textbox1.Text;
Dictionary<string, string> map = new Dictionary<string, string>();

map.Add("AB", "x");
map.Add("Ab", "z");
map.Add("aB", "p");

string temp = input;
foreach (var entry in map)
{
    string key = entry.Key;
    string value = entry.Value;
    temp = Regex.Replace(temp, key, match =>
    {
        bool isUpper = char.IsUpper(match.Value[0]);

        char[] result = value.ToCharArray();
        result[0] = isUpper
            ? char.ToUpper(result[0])
            : char.ToLower(result[0]);
        return new string(result);
    }, RegexOptions.IgnoreCase);

}

This is my code...I use this for parse my small words. Like AB=x CB=z etc etc...

But my problem is - it can't understand lowecase and upercase (not case sensative)

Look I mapped :

map.Add("AB", "x");
map.Add("Ab", "z");
map.Add("aB", "p");

I mapped every AB,Ab,aB for different result.

But when i put AB,Ab,aB it just shows "x" !

But i want to show different result for eaches.

Where is my coding problem.

What I have tried:

map.Add("AB", "x");
map.Add("Ab", "z");
map.Add("aB", "p");