tramp-file-name-regexp

tramp-file-name-regexp is a variable defined in `tramp.el'.
Its value is

"\\`/[^/|:][^/|]*:"

• This variable may be risky if used as a file-local variable.

Documentation:
Regular expression matching file names handled by Tramp.
This regexp should match Tramp file names but no other file names.
When tramp.el is loaded, this regular expression is prepended to
`file-name-handler-alist', and that is searched sequentially. Thus,
if the Tramp entry appears rather early in the `file-name-handler-alist'
and is a bit too general, then some files might be considered Tramp
files which are not really Tramp files.

Please note that the entry in `file-name-handler-alist' is made when
this file (tramp.el) is loaded. This means that this variable must be set
before loading tramp.el. Alternatively, `file-name-handler-alist' can be
updated after changing this variable.

Also see `tramp-file-name-structure'.