Use selected region as search value when starting isearch

init.el

@@ -810,6 +810,18 @@ debian, and derivatives). On most it's 'fd'.")
             "M-c" 'mdrp/toggle-case
             "c"    'mdrp/toggle-case)
   :config
+  ;; use current region as first search
+  (defadvice isearch-mode (around isearch-mode-default-string (forward &optional regexp op-fun recursive-edit word-p) activate)
+    (if (and transient-mark-mode mark-active (not (eq (mark) (point))))
+        (progn
+          (isearch-update-ring (buffer-substring-no-properties (mark) (point)))
+          (deactivate-mark)
+          ad-do-it
+          (if (not forward)
+              (isearch-repeat-backward)
+            (goto-char (mark))
+            (isearch-repeat-forward)))
+      ad-do-it))
   (defun mdrp/toggle-case ()
     (interactive)
     (setq case-fold-search (not case-fold-search))

init.org

@@ -1502,6 +1502,18 @@ This part will load packages that are useful for emacs and are not specific to m
               "M-c" 'mdrp/toggle-case
               "c"    'mdrp/toggle-case)
     :config
+    ;; use current region as first search
+    (defadvice isearch-mode (around isearch-mode-default-string (forward &optional regexp op-fun recursive-edit word-p) activate)
+      (if (and transient-mark-mode mark-active (not (eq (mark) (point))))
+          (progn
+            (isearch-update-ring (buffer-substring-no-properties (mark) (point)))
+            (deactivate-mark)
+            ad-do-it
+            (if (not forward)
+                (isearch-repeat-backward)
+              (goto-char (mark))
+              (isearch-repeat-forward)))
+        ad-do-it))
     (defun mdrp/toggle-case ()
       (interactive)
       (setq case-fold-search (not case-fold-search))