how to replace richtextbox.rtf if and only if the whole word match?

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C#

VS2012

rtTextArea.Rtf = rtTextArea.Rtf.Replace(oldtext, newtext);

but the problem is it replaces all occurence of the words instead of the required word e.g origianl word :- hello my name is serak and am in israel replace() -- is with are the whole word chages to "are serak and am in arereal"

is there anyway i can include a condition or something to match the whole word? my working environment C#.