register-read-with-preview

register-read-with-preview is a compiled Lisp function in `register.el'.

(register-read-with-preview PROMPT)

Read and return a register name, possibly showing existing registers.
Prompt with the string PROMPT. If `register-alist' and
`register-preview-delay' are both non-nil, display a window
listing existing registers after `register-preview-delay' seconds.
If `help-char' (or a member of `help-event-list') is pressed,
display such a window regardless.