[Saioko]

I had Calibre for several years, it worked ok. But today it simply refused to start up. There were no changes - I didn't update Calibre or install any updates before the failure, then I tried updating Calibre and installing latest updates for my Linux, but nothing changed.
Here is what I get:

calibre, version 2.99.9
Traceback (most recent call last):
  File "/home/katya/calibre/src/calibre/gui2/main.py", line 292, in initialize_db_stage2
    self.start_gui(db)
  File "/home/katya/calibre/src/calibre/gui2/main.py", line 227, in start_gui
    main = self.main = Main(self.opts, gui_debug=self.gui_debug)
  File "/home/katya/calibre/src/calibre/gui2/ui.py", line 160, in __init__
    ac = self.init_iaction(action)
  File "/home/katya/calibre/src/calibre/gui2/ui.py", line 174, in init_iaction
    ac = action.load_actual_plugin(self)
  File "/home/katya/calibre/src/calibre/customize/__init__.py", line 614, in load_actual_plugin
    ac = getattr(importlib.import_module(mod), cls)(gui,
  File "importlib/__init__.py", line 37, in import_module
  File "/home/katya/calibre/src/calibre/gui2/actions/catalog.py", line 13, in <module>
    from calibre.gui2.tools import generate_catalog
  File "/home/katya/calibre/src/calibre/gui2/tools.py", line 16, in <module>
    from calibre.gui2.convert.single import NoSupportedInputFormats
  File "/home/katya/calibre/src/calibre/gui2/convert/single.py", line 20, in <module>
    from calibre.gui2.convert.search_and_replace import SearchAndReplaceWidget
  File "/home/katya/calibre/src/calibre/gui2/convert/search_and_replace.py", line 11, in <module>
    from calibre.gui2.convert.search_and_replace_ui import Ui_Form
  File "/home/katya/calibre/src/calibre/gui2/convert/search_and_replace_ui.py", line 155, in <module>
    from regex_builder import RegexEdit
  File "/home/katya/calibre/src/calibre/gui2/convert/regex_builder.py", line 18, in <module>
    from calibre.ebooks.conversion.search_replace import compile_regular_expression
  File "/home/katya/calibre/src/calibre/ebooks/conversion/search_replace.py", line 7, in <module>
    import regex
  File "site-packages/regex/__init__.py", line 383, in <module>
  File "site-packages/regex/_regex_core.py", line 21, in <module>
  File "/home/katya/calibre/src/calibre/constants.py", line 209, in __getitem__
    raise KeyError('No plugin named %r'%name)
KeyError: "No plugin named '_regex'"