There are two possible problems. The calibre interface and the syntax of regular expressions. If you don't already know regular expressions, then I would suggest that is where you should start. There are books and whole websites about it. And also several posts here that tries to explain specific regexp search and replaces.
The interface is simple and doesn't differ from common practice. You create a regexp to match a field, with parentheses to be able to reference parts, and replace with a string that may include references to the parts designated with parentheses.
I assume that you have read what the manual has to say about it:
http://manual.calibre-ebook.com/regexp.html