How to replace all occurrences of a string with a specified string from rich text box in c#?

I have a rich text box and want to replace multiple occurrences of a string in a rich text box with a specified string. Currently i am playing with this but nothing is happening with this:

 this.textContainer_rtb.Text = Regex.Replace(this.textContainer_rtb.Text, OldWord , NewWord);

Please Help?