Added code

ui/modules/validators/page_index.py

@@ -0,0 +1,14 @@
+from ui.modules.updaters.page_index import update_page_text
+from ui.gui import Ui_MainWindow
+from PyQt5 import QtGui
+
+
+def validate_page_index(ui: Ui_MainWindow, event: QtGui.QKeyEvent, old_event):
+    old_event(event)
+    for char in ui.page_index.text():
+        if char not in '0123456789':
+            ui.page_index.setText(''.join(c for c in ui.page_index.text() if c in '0123456789'))
+            ui.page_index.setCursorPosition(QtGui.QTextCursor.End)
+            break
+
+    update_page_text(ui)