* lisp/replace.el (read-regexp-defaults-function): New defcustom.

* lisp/replace.el (read-regexp-defaults-function): New defcustom.
(read-regexp-suggestions): New function.
(read-regexp): Use `read-regexp-defaults-function' to get default values.
Use `read-regexp-suggestions'.  Add non-empty default to history
for empty input.
(occur-read-regexp-defaults-function): Remove function.
(occur-read-primary-args): Use `regexp-history-last' instead of
`occur-read-regexp-defaults-function'.

* lisp/hi-lock.el (hi-lock-read-regexp-defaults-function): Remove function.
(hi-lock-line-face-buffer, hi-lock-face-buffer)
(hi-lock-face-phrase-buffer): Use `regexp-history-last' instead of
`hi-lock-read-regexp-defaults-function'.  Doc fix.
(hi-lock-face-symbol-at-point): Replace `find-tag-default-as-regexp'
with `find-tag-default-as-symbol-regexp'.  Doc fix.
(hi-lock-read-regexp-defaults): Remove function.
(hi-lock-regexp-okay): Add check for null.

* lisp/progmodes/grep.el (grep-read-regexp): Use `grep-tag-default' for
the arg DEFAULTS.  Move formatting of the prompt to `read-regexp'.

* lisp/subr.el (find-tag-default-as-symbol-regexp): New function.
(find-tag-default-as-regexp): Move symbol regexp formatting to
`find-tag-default-as-symbol-regexp'.