Replace single backslash "\" with double backslashes "\\"

var replaceableString = "c:\asd\flkj\klsd\ffjkl";
alert(replaceableString);

This will alert you c:asdlkjklsdfjkl because '\' is an escape character which will not be considered.

To have a backslash in your string , you should do something like this..

var replaceableString = "c:\\asd\\flkj\\klsd\\ffjkl";
alert(replaceableString);

This will alert you c:\asd\flkj\klsd\ffjkl

JS Fiddle

Learn about Escape sequences here

If you want your string to have '\' by default , you should escape it .. Use escape() function

var replaceableString = escape("c:\asd\flkj\klsd\ffjkl");
alert(replaceableString);

JS Fiddle

Try:

   var parts = replaceableString.split('\\');
   var output = parts.join('\\\\');

Personally, as I am not so expert in reg exps, I tend to avoid them when dealing with non-alphanumeric characters, both due to readability and to avoid weird mistake.