display-format-alist

display-format-alist is a variable defined in `frame.el'.
Its value is

(("\\`[^:]*:[0-9]+\\(\\.[0-9]+\\)?\\'" . x))

Documentation:
Alist of patterns to decode display names.
The car of each entry is a regular expression matching a display
name string. The cdr is a symbol giving the window-system that
handles the corresponding kind of display.