How can I replace forward slash with backslash in a variable in a qmake .pro file?

In a .pro file, I am trying to replace the forward slashes in, e.g., the PWD variable, with backslashes. I am working in Qt Creator on Windows.

The following code will correctly replace the D drive with the C drive:

currdir = $$PWDcurrdir ~= s/D:/C:/

The following code gives an error that "the s/// function expects 3 or 4 arguments"

currdir = $$PWDcurrdir ~= s/\//\\/