I'm not sure that regular expressions are the correct tool for this. Is there any reason why you can't just do:
String myString = "This U:'0 is a U+'0 string."
myString = myString.Replace(":'", "'");
myString = myString.Replace(":+", "+");
myString = myString.Replace("+'", "'");
Unless the logic behind the transformations is far more involved than you are actually suggesting, there's no reason why this wouldn't be sufficient.