Replace text while keeping case intact in C#

Question

I have a set of sentences i need to use to do a replace, for example:

abc => cde
ab df => de
...

And i have a text where to make the chan

Answer 1

Not sure how well this will work, but this is what I came up with:

        string input = "A bgt abc hyi. Abc Ab df h";
        Dictionary map = new Dictionary();
        map.Add("abc", "cde");
        map.Add("ab df", "de");

        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);
        }
        label1.Text = temp; // output is A bgt cde hyi. Cde De h

EDIT After reading the modified question, here's my modified code (it turns out to be similar steps to @Sephallia's code.. and similar variable names lol )

The code now is a bit more complicated.. but I think it's ok

        string input = 
        @"As he didn't achieve success, he was fired.
        As he DIDN'T ACHIEVE SUCCESS, he was fired.
        As he Didn't Achieve Success, he was fired.
        As he Didn't achieve success, he was fired.";
        Dictionary map = new Dictionary();
        map.Add("didn't achieve success", "failed miserably");


        string temp = input;
        foreach (var entry in map)
        {
            string key = entry.Key;
            string value = entry.Value;
            temp = Regex.Replace(temp, key, match =>
            {
                bool isFirstUpper, isEachUpper, isAllUpper;

                string sentence = match.Value;
                char[] sentenceArray = sentence.ToCharArray();

                string[] words = sentence.Split(' ');

                isFirstUpper = char.IsUpper(sentenceArray[0]);

                isEachUpper = words.All(w => char.IsUpper(w[0]) || !char.IsLetter(w[0]));

                isAllUpper = sentenceArray.All(c => char.IsUpper(c) || !char.IsLetter(c));

                if (isAllUpper)
                    return value.ToUpper();

                if (isEachUpper)
                {
                    // capitalize first of each word... use regex again :P
                    string capitalized = Regex.Replace(value, @"\b\w", charMatch => charMatch.Value.ToUpper());
                    return capitalized;
                }


                char[] result = value.ToCharArray();
                result[0] = isFirstUpper
                    ? char.ToUpper(result[0])
                    : char.ToLower(result[0]);
                return new string(result);
            }, RegexOptions.IgnoreCase);
        }
        textBox1.Text = temp; 
        /* output is :
        As he failed miserably, he was fired.
        As he FAILED MISERABLY, he was fired.
        As he Failed Miserably, he was fired.
        As he Failed miserably, he was fired.
        */