isearch-backward-regexp

isearch-backward-regexp is an interactive compiled Lisp function in `isearch.el'.

It is bound to C-M-r, <menu-bar> <edit> <search> <i-search> <isearch-backward-regexp>.

(isearch-backward-regexp &optional NOT-REGEXP NO-RECURSIVE-EDIT)

Do incremental search backward for regular expression.
With a prefix argument, do a regular string search instead.
Like ordinary incremental search except that your input is treated
as a regexp. See the command `isearch-forward' for more information.