Test if URLs are recognized as filenames by commands such as "gf". Here
we'll use `expand("<cfile>")` since "gf" would need to open the file.

Insert some URLs:

  % first test for URL://machine.name/tmp/vimtest2a and other text<cr>
  % second test for URL://machine.name/tmp/vimtest2b. And other text<cr>
  % third test for URL:\\machine.name\vimtest2c and other text<cr>
  % fourth test for URL:\\machine.name\tmp\vimtest2d, and other text

Go to the first URL:

  :/^first<cr>/tmp

Append the url to the beginning of the buffer:

  :call append(0, expand("<cfile>"))

Repeat for the second URL, but navigate to the 'URL' word:

  :/^second<cr>/URL<cr>:call append(1, expand("<cfile>"))

Repeat for the other two, but first the 'isfname' option must be set to
allow '\' in filenames

  :set isf=@,48-57,/,.,-,_,+,,,$,:,~,\
  :/^third<cr>/name<cr>:call append(2, expand("<cfile>"))
  :/^fourth<cr>/URL<cr>:call append(3, expand("<cfile>"))

Delete the initial text which starts at line 5:

  > 5GdG

Result:

  URL://machine.name/tmp/vimtest2a
  URL://machine.name/tmp/vimtest2b
  URL:\\machine.name\vimtest2c
  URL:\\machine.name\tmp\vimtest2d
  @end