Changing a macro value thru regular expressions

[Alfetta159]

I'd like to be able to change the value of an existing macro using regular expression to in effect do a search and replace, but I'm not sure if that can be done with the set macro step. I mean, I can setup the existing macro value, but I dont' know how to get the changed value into the macro.

I guess I mean, I know how to do the search, but not the replace.