replace hyphen

I am trying to replace a hyphen from a Grade attribute by using the Replace operator. I would like to replace it with text that describes no value has been entered (i.e., Not indicated). The problem is that the attribute includes values such as - (the hyphen I want to replace), A-, B-, C-. Using the replace operator replaces all of the hyphens (including those being used as minuses). I tried using the regular expression, \b[-]\b, but that is not working. I also tried, \b["-"]\b without success.