ImportError: No module named regex

When uploading a new document, I receive this error:

[2018-01-04 04:24:50,064: ERROR/MainProcess] Task ocr.tasks.task_do_ocr[1f44c0b3-64de-4ba1-b4cf-1972159c682b] raised unexpected: ImportError('No module named regex',)
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/celery/app/trace.py", line 240, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/celery/app/trace.py", line 438, in __protected_call__
    return self.run(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/mayan/apps/ocr/tasks.py", line 43, in task_do_ocr
    document_version=document_version
  File "/usr/local/lib/python2.7/dist-packages/mayan/apps/ocr/managers.py", line 56, in process_document_version
    sender=document_version.__class__, instance=document_version
  File "/usr/local/lib/python2.7/dist-packages/django/dispatch/dispatcher.py", line 191, in send
    response = receiver(signal=self, sender=sender, **named)
  File "/usr/local/lib/python2.7/dist-packages/mayan/apps/document_analyzer/handlers.py", line 12, in analyze_document
    Result.objects.process_version(document_version=instance)
  File "/usr/local/lib/python2.7/dist-packages/mayan/apps/document_analyzer/managers.py", line 19, in process_version
    analyzer_class = import_string(analyzer.type)
  File "/usr/local/lib/python2.7/dist-packages/django/utils/module_loading.py", line 20, in import_string
    module = import_module(module_path)
  File "/usr/lib/python2.7/importlib/__init__.py", line 37, in import_module
    __import__(name)
  File "/usr/local/lib/python2.7/dist-packages/mayan/apps/document_analyzer/backends/datefind.py", line 6, in <module>
    import regex as re
ImportError: No module named regex

I am using the Docker base image like last time. https://gitlab.com/jerrac/mayan-document_analyzer_not_working_example

I'm guessing that I'm missing a dependency in the Docker image?