Variable: file-name-handler-alist

Alist of elements (REGEXP . HANDLER) for file names handled specially.
If a file name matches REGEXP, all I/O on that file is done by calling
HANDLER. If a file name matches more than one handler, the handler
whose match starts last in the file name gets precedence. The
function `find-file-name-handler' checks this list for a handler for
its argument.

HANDLER should be a function. The first argument given to it is the
name of the I/O primitive to be handled; the remaining arguments are
the arguments that were passed to that primitive. For example, if you
do (file-exists-p FILENAME) and FILENAME is handled by HANDLER, then
HANDLER is called like this:

(funcall HANDLER 'file-exists-p FILENAME)

Note that HANDLER must be able to handle all I/O primitives; if it has
nothing special to do for a primitive, it should reinvoke the
primitive to handle the operation "the usual way". See Info node `(elisp)Magic File Names' for more details.