diff --git a/src/calibre/gui2/tweak_book/function_replace.py b/src/calibre/gui2/tweak_book/function_replace.py
index 2bdfff212d..a81845c3ea 100644
--- a/src/calibre/gui2/tweak_book/function_replace.py
+++ b/src/calibre/gui2/tweak_book/function_replace.py
@@ -14,7 +14,8 @@ from qt.core import QApplication, QDialogButtonBox, QFontMetrics, QHBoxLayout, Q
 from calibre.ebooks.oeb.polish.utils import apply_func_to_html_text, apply_func_to_match_groups
 from calibre.gui2 import error_dialog
 from calibre.gui2.complete2 import EditWithComplete
-from calibre.gui2.tweak_book import dictionaries
+from calibre.gui2.dialogs.confirm_delete import confirm
+from calibre.gui2.tweak_book import dictionaries, tprefs
 from calibre.gui2.tweak_book.editor.text import TextEdit
 from calibre.gui2.tweak_book.widgets import Dialog
 from calibre.utils.config import JSONConfig
@@ -253,6 +254,7 @@ class FunctionEditor(Dialog):
         l.addWidget(la)
 
         l.addWidget(self.bb)
+        self.initial_source = self.source
 
     def sizeHint(self):
         fm = QFontMetrics(self.font())
@@ -285,6 +287,13 @@ class FunctionEditor(Dialog):
 
         Dialog.accept(self)
 
+    def reject(self):
+        if self.source != self.initial_source:
+            if not confirm(_('All unsaved changes will be lost. Are you sure?'), 'function-replace-close-confirm',
+                           parent=self, config_set=tprefs):
+                return
+        return super().reject()
+
 # Builtin functions ##########################################################