Similar to :bind, you can use :mode and
:interpreter to establish a deferred binding within the
auto-mode-alist and interpreter-mode-alist variables
(see Auto Major Mode in GNU Emacs Lisp Reference Manual).
The specifier to either keyword can be a cons cell, a list of cons
cells, or a string or regexp.
The following example reproduces the default ruby-mode
configuration, exactly as it is in Emacs out-of-the-box. That mode is
enabled automatically when a file whose name matches the regexp
"\\.rb\\'" (a file with the .rb extension), or when the
first line of the file (known as the “shebang”) matches the string
"ruby":
(use-package ruby-mode :mode "\\.rb\\'" :interpreter "ruby")
The default python-mode configuration can be reproduced using
the declaration below. Note that the package that should be loaded
differs from the mode name in this case, so we must use a cons:
;; The package is "python" but the mode is "python-mode":
(use-package python
:mode ("\\.py\\'" . python-mode)
:interpreter ("python" . python-mode))
Both the :mode and :interpreter keywords also accept a
list of regexps:
(use-package foo
;; Equivalent to "\\(ba[rz]\\)\\'":
:mode ("\\.bar\\'" "\\.baz\\'")
;; Equivalent to "\\(foo[ab]\\)":
:interpreter ("fooa" "foob"))