E-book viewer: Show an error if the user tries to search for only punctuation or spaces in the search modes that ignore these. Fixes #2015795 [Searching for punctuation freezes the e-book viewer](https://bugs.launchpad.net/calibre/+bug/2015795)

src/calibre/gui2/viewer/search.py

@@ -145,6 +145,14 @@ class Search:
             self._nsd = word_pats, full_pat
         return self._nsd
 
+    @property
+    def is_empty(self):
+        if not self.text:
+            return True
+        if self.mode in ('normal', 'word') and not regex.sub(r'[\s\p{P}]+', '', self.text):
+            return True
+        return False
+
     def __str__(self):
         from collections import namedtuple
         s = ('text', 'mode', 'case_sensitive', 'backwards')

src/calibre/gui2/viewer/ui.py

@@ -336,6 +336,9 @@ class EbookViewer(MainWindow):
     def start_search(self, search_query):
         name = self.web_view.current_content_file
         if name:
+            if search_query.is_empty and search_query.text:
+                return error_dialog(self, _('Empty search expression'), _(
+                    'Cannot search for {!r} as it contains only punctuation and spaces.').format(search_query.text), show=True)
             self.web_view.get_current_cfi(self.search_widget.set_anchor_cfi)
             self.search_widget.start_search(search_query, name)
             self.web_view.setFocus(Qt.FocusReason.OtherFocusReason)