I'm running the following code on a string:

strSystemUserUUIDF1 = Regex.Replace(strSystemUserUUID, "\b%7B\b", "{");

strSystemUserUUIDF2 = Regex.Replace(strSystemUserUUIDF1, "\b%7D\b", "}");

strSystemUserUUIDF3 = Regex.Replace(strSystemUserUUIDF2, "\b%2D\b", "-");

I was expecting %7B to be replaced with { and so on.

Nothing is being replaced!

I was originally doing everything with just one string
(strSystemUserUUID), but now I'm trying as above out of desperation. 
Nothing works though. Anyone have any idea why?