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diff --git a/tests/test-custom-ordering--region-helpers.el b/tests/test-custom-ordering--region-helpers.el
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+;;; test-custom-ordering--region-helpers.el --- Tests for the shared ordering region helpers -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; cj/--ordering-validate-region and cj/--ordering-replace-region were extracted
+;; from the seven pure ordering helpers (the copy-pasted start>end guard) and the
+;; interactive ordering commands (the copy-pasted delete-region + insert tail).
+;; The per-command behavior stays covered by the existing wrapper/transform
+;; tests; these cover the extracted helpers directly.
+
+;;; Code:
+
+(require 'ert)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'custom-ordering)
+
+;;; cj/--ordering-validate-region
+
+(ert-deftest test-custom-ordering-validate-region-accepts-ordered ()
+  "Normal: start < end returns nil without signalling."
+  (should (null (cj/--ordering-validate-region 1 10))))
+
+(ert-deftest test-custom-ordering-validate-region-accepts-equal ()
+  "Boundary: start = end (empty region) is allowed."
+  (should (null (cj/--ordering-validate-region 5 5))))
+
+(ert-deftest test-custom-ordering-validate-region-rejects-inverted ()
+  "Error: start > end signals with both positions in the message."
+  (let ((err (should-error (cj/--ordering-validate-region 10 3) :type 'error)))
+    (should (string-match-p "10" (error-message-string err)))
+    (should (string-match-p "3" (error-message-string err)))))
+
+;;; cj/--ordering-replace-region
+
+(ert-deftest test-custom-ordering-replace-region-swaps-text ()
+  "Normal: the region between START and END is replaced with INSERTION and
+point is left at START."
+  (with-temp-buffer
+    (insert "AAAABBBB")
+    (cj/--ordering-replace-region 1 5 "xx")   ; replace the first AAAA
+    (should (equal "xxBBBB" (buffer-string)))
+    (should (= (point) 3))))                  ; START (1) + len("xx")
+
+(ert-deftest test-custom-ordering-replace-region-empty-insertion ()
+  "Boundary: an empty INSERTION just deletes the region."
+  (with-temp-buffer
+    (insert "keepDROP")
+    (cj/--ordering-replace-region 5 9 "")     ; drop "DROP" (positions 5-8)
+    (should (equal "keep" (buffer-string)))))
+
+(provide 'test-custom-ordering--region-helpers)
+;;; test-custom-ordering--region-helpers.el ends here