How to search and replace exact matching strings only

//Find and Replace A word in c#
public static class Program
    {
        public static string Replace(this String str, char[] chars, string replacement)
        {
            StringBuilder output = new StringBuilder(str.Length);
            bool replace = false;
            if (chars.Length - 1 < 1)
            {
                for (int i = 0; i < str.Length; i++)
                {

                    char c = str[i];
                    replace = false;
                    //  int val = Regex.Matches(ch.ToString(), @"[a-zA-Z]").Count;


                    for (int j = 0; j < chars.Length; j++)
                    {
                        if (chars[j] == c)
                        {
                            replace = true;

                            break;

                        }

                    }

                    if (replace)
                        output.Append(replacement);
                    else
                        output.Append(c);
                }
            }
            else
            {

                int j = 0;
                int truecount = 0;
                    char c1 = '\0';
                    for (int k = 0; k < str.Length; k++)
                    {
                       c1 = str[k];

                        if (chars[j] == c1)
                        {
                            replace = true;
                            truecount ++;


                        }

                        else
                        {
                            truecount = 0;
                            replace = false;
                            j = 0;
                        }
                        if(truecount>0)
                        {
                            j++;
                        }

                        if (j > chars.Length-1)
                        {
                            j = 0;
                        }

                        if (truecount == chars.Length)
                        {
                            output.Remove(output.Length - chars.Length+1, chars.Length-1);
                           // output.Remove(4, 2);
                            if (replace)
                                output.Append(replacement);
                            else
                                output.Append(c1);
                        }
                        else
                        {
                            output.Append(c1);
                        }


                    }

            }


            return output.ToString();
        }


    static void Main(string[] args)
    {

        Console.WriteLine("Enter a word");
        string word = (Console.ReadLine());

        Console.WriteLine("Enter a word to find");
        string find = (Console.ReadLine());

        Console.WriteLine("Enter a word to Replace");
        string Rep = (Console.ReadLine());

        Console.WriteLine(Replace(word, find.ToCharArray(), Rep));
        Console.ReadLine();

    }

}

string pattern = @"\bAdd\b";
string input = "Add Additional String to text box";
string result = Regex.Replace(input, pattern, "Insert", RegexOptions.None);  

"\bAdd\b" ensures that it will match the "Add" which is not part of other words. Hope it's helpful.

answer for:
"This solution doesnt work if I need to replace a word that starts with @. Fiddle here dotnetfiddle.net/9kgW4h How can I get this working in this scenario. – Frenz Jan 16 '17 at 5:46"

possible solution:

public static string SafeReplace(this string input, string find, string replace, bool matchWholeWord) {
    string searchString = find.StartsWith("@") ? $@"@\b{find.Substring(1)}\b" : $@"\b{find}\b"; 
    string textToFind = matchWholeWord ? searchString : find;
    return Regex.Replace(input, textToFind, replace);
} 

You can use Regex to do this:

Extension method example:

public static class StringExtensions
{
    public static string SafeReplace(this string input, string find, string replace, bool matchWholeWord)
    {
        string textToFind = matchWholeWord ? string.Format(@"\b{0}\b", find) : find;
        return Regex.Replace(input, textToFind, replace);
    }
}

Usage:

  string text = "Add Additional String to text box";
  string result = text.SafeReplace("Add", "Insert", true);

result: "Insert Additional String to text box"