diff --git a/git-auto-commit-mode.el b/git-auto-commit-mode.el
index d5f8afc..00ef4fe 100644
--- a/git-auto-commit-mode.el
+++ b/git-auto-commit-mode.el
@@ -36,6 +36,8 @@
 
 ;;; Code:
 
+(require 'subr-x)
+
 (defgroup git-auto-commit-mode nil
   "Customization options for `git-auto-commit-mode'."
   :group 'external)
@@ -118,14 +120,9 @@ It can be:
 (defun gac-relative-file-name (filename)
   "Find the path to FILENAME relative to the git directory."
   (let* ((git-dir
-          (replace-regexp-in-string
-           "\n+$" "" (shell-command-to-string
-                      "git rev-parse --show-toplevel")))
-         (relative-file-name
-          (replace-regexp-in-string
-           "^/" "" (replace-regexp-in-string
-                    git-dir "" filename))))
-    relative-file-name))
+          (string-trim-right
+           (shell-command-to-string "git rev-parse --show-toplevel"))))
+    (file-relative-name filename git-dir)))
 
 (defun gac-password (proc string)
   "Ask the user for a password when necessary.
diff --git a/tests/git-auto-commit-mode-tests.el b/tests/git-auto-commit-mode-tests.el
index fd40dce..2b0b959 100644
--- a/tests/git-auto-commit-mode-tests.el
+++ b/tests/git-auto-commit-mode-tests.el
@@ -64,5 +64,104 @@
                       :not :to-match (rx string-start "test" string-end)))
           (delete-directory temp-dir t))))))
 
+(describe "Getting a relative path"
+  (it "should return the file name for files in the project root"
+    (let* ((temp-dir (make-temp-file "gac-" t))
+           (temp-file (expand-file-name "test" temp-dir))
+           (default-directory temp-dir))
+      (unwind-protect
+          (progn
+            (shell-command "git init")
+            (let ((buffer (find-file-noselect temp-file)))
+              (with-current-buffer buffer
+                (git-auto-commit-mode)
+                (insert "test")
+                (save-buffer)))
+            (expect (gac-relative-file-name temp-file)
+                    :to-equal "test"))
+        (delete-directory temp-dir t))))
+
+  (it "should return the file name and directory name for nested files"
+    (let* ((temp-dir (make-temp-file "gac-" t))
+           (temp-file (expand-file-name "test/test.txt" temp-dir))
+           (default-directory temp-dir))
+      (mkdir (expand-file-name "test" temp-dir))
+      (unwind-protect
+          (progn
+            (shell-command "git init")
+            (let ((buffer (find-file-noselect temp-file)))
+              (with-current-buffer buffer
+                (git-auto-commit-mode)
+                (insert "test")
+                (save-buffer)))
+            (expect (gac-relative-file-name temp-file)
+                    :to-equal "test/test.txt"))
+        (delete-directory temp-dir t))))
+
+  (it "should handle ‘[]’ in the filename"
+    (let* ((temp-dir (make-temp-file "gac-" t))
+           (temp-file (expand-file-name "[test]-test.txt" temp-dir))
+           (default-directory temp-dir))
+      (unwind-protect
+          (progn
+            (shell-command "git init")
+            (let ((buffer (find-file-noselect temp-file)))
+              (with-current-buffer buffer
+                (git-auto-commit-mode)
+                (insert "test")
+                (save-buffer)))
+            (expect (gac-relative-file-name temp-file)
+                    :to-equal "[test]-test.txt"))
+        (delete-directory temp-dir t))))
+
+  (it "should handle ‘[]’ in the directory name"
+    (let* ((temp-dir (make-temp-file "gac-" t))
+           (temp-file (expand-file-name "[test]-test/testing.txt" temp-dir))
+           (default-directory temp-dir))
+      (mkdir (expand-file-name "[test]-test" temp-dir))
+      (unwind-protect
+          (progn
+            (shell-command "git init")
+            (let ((buffer (find-file-noselect temp-file)))
+              (with-current-buffer buffer
+                (git-auto-commit-mode)
+                (insert "test")
+                (save-buffer)))
+            (expect (gac-relative-file-name temp-file)
+                    :to-equal "[test]-test/testing.txt"))
+        (delete-directory temp-dir t))))
+
+  (it "should handle ‘[’ in the directory name"
+    (let* ((temp-dir (make-temp-file "gac-[test-" t))
+           (temp-file (expand-file-name "testing.txt" temp-dir))
+           (default-directory temp-dir))
+      (unwind-protect
+          (progn
+            (shell-command "git init")
+            (let ((buffer (find-file-noselect temp-file)))
+              (with-current-buffer buffer
+                (git-auto-commit-mode)
+                (insert "test")
+                (save-buffer)))
+            (expect (gac-relative-file-name temp-file)
+                    :to-equal "testing.txt"))
+        (delete-directory temp-dir t))))
+
+  (it "should handle ‘[’ in the file name"
+    (let* ((temp-dir (make-temp-file "gac-" t))
+           (temp-file (expand-file-name "[test-test.txt" temp-dir))
+           (default-directory temp-dir))
+      (unwind-protect
+          (progn
+            (shell-command "git init")
+            (let ((buffer (find-file-noselect temp-file)))
+              (with-current-buffer buffer
+                (git-auto-commit-mode)
+                (insert "test")
+                (save-buffer)))
+            (expect (gac-relative-file-name temp-file)
+                    :to-equal "[test-test.txt"))
+        (delete-directory temp-dir t)))))
+
 (provide 'git-auto-commit-mode-tests)
 ;;; git-auto-commit-mode-tests.el ends here