1) item returns a single slash,
or it could be another preprocessing step that returns single slash, like Javascript, e.g.:

return "\\";

2) Setting second Replace step:

\ -> \\
Result: \

3) Setting second Replace step:

\\ -> \\
Result: \

4) Setting second Replace step:

\\\ -> \\
Result: \

5) Setting second Replace step:

\\\ -> \\\
Result: \

6) Setting second Replace step:

\\\\ -> \\\\
Result: \