How to replace inside regex?

[Suliman via Digitalmars-d-learn]

I can't understand how to replace in regex. I have got next task:
find all commas in strings inside quotes and replace them.
foo, bar, "hello, user", baz

I wrote next regexp that find part that include commas inside the 
quotes:
auto partWithComma = matchAll(line, r);

but I can't understand how to replace commas here. I have only 
ideas to do something like:

auto partWithComma = matchAll(line, r).replaceAll(",", " ");