Error occured when disable auto dereference and use StringField regex check

import unittest
from mongoengine import DynamicDocument, connect
from mongoengine.fields import StringField



class User(DynamicDocument):
    username = StringField(required=True)
    pa_id = StringField(required=True, regex=r'(^PA\d\d\d\d$)')


class Test(unittest.TestCase):
    def setUp(self) -> None:
        connection = connect(db='mongotest', host='mongomock://localhost')

    def test(self):
        a = User(username='stonemoe', pa_id='PA0001').save()
        self.assertEqual(1, User.objects.count())

        # RAISE HERE: TypeError: cannot deepcopy this pattern object
        for user in User.objects.no_dereference():
            print(user)
        

if __name__ == "__main__":
    suite = unittest.defaultTestLoader.loadTestsFromTestCase(Test)
    unittest.TextTestRunner().run(suite)

Root cause may be:
https://github.com/MongoEngine/mongoengine/blob/92f6fce77d12f930333bd8980fa611627d3a8d1b/mongoengine/base/document.py#L782-L783

Start with the provided unittest reproduction and inspect mongoengine/base/document.py around lines 782-783, where the issue identifies the possible root cause. Verify the failure when iterating over User.objects.no_dereference() with the StringField regex, then confirm that the test completes without the deepcopy TypeError.