When you use the minibuffer, C-r is not bound to isearch-backward. You are not using Isearch at that time, and Isearch does not use the minibuffer, even though it might look like it does.

Perhaps you really mean M-r in the minibuffer, which is previous-matching-history-element and which completes against the current minibuffer input history.

In Isearch you can use M-TAB (or C-M-i or ESC TAB - useful if your window manager captures M-TAB) to match a past search string using completion.

If you use library Isearch+ or Icicles then M-TAB provides better completion.

When you use the minibuffer, C-r is not bound to isearch-backward. You are not using Isearch at that time, and Isearch does not use the minibuffer, even though it might look like it does.

Perhaps you really mean M-r in the minibuffer, which is previous-matching-history-element and which completes against the current minibuffer input history.

In Isearch you can use M-TAB (or C-M-i or ESC TAB - useful if your window manager captures M-TAB) to match a past search string using completion.

(In Isearch M-r is bound to isearch-toggle-regexp. You can of course change which keys are bound to which Isearch commands, by using define-key with isearch-mode-map.)

If you use library Isearch+ or Icicles then M-TAB provides better completion against the Isearch search-string histories (search-ring and regexp-search-ring).

When you use the minibuffer, C-r is not bound to isearch-backward (you are not using Isearch at that time, and Isearch does not use the minibuffer).

You can use M-TAB (or C-M-i or ESC TAB - useful if your window manager captures M-TAB) to match a past search string using completion.

If you use library Isearch+ or Icicles then M-TAB provides better completion.

When you use the minibuffer, C-r is not bound to isearch-backward. You are not using Isearch at that time, and Isearch does not use the minibuffer, even though it might look like it does.

Perhaps you really mean M-r in the minibuffer, which is previous-matching-history-element and which completes against the current minibuffer input history.

In Isearch you can use M-TAB (or C-M-i or ESC TAB - useful if your window manager captures M-TAB) to match a past search string using completion.

If you use library Isearch+ or Icicles then M-TAB provides better completion.