Len() of unsized object Error when loading ImagDataBunch with fastai

I had run this code with a very simple regular expression as you can see /d{4} . But i am getting a error which says Failed to find {pat} in the log. Does this mean my reg expression is wrong?. Also the error says len() of unsized object. I am not sure what caused this error. Pls let me know where i went wrong. Thanks for any help

%reload_ext autoreload
%autoreload 2
%matplotlib inline

from fastai.vision import *
from fastai.metrics import error_rate
from mat4py import loadmat
from pylab import*
import matplotlib
import os
import numpy as np # linear algebra
import pandas as pd
import matplotlib.pyplot as plt
import matplotlib.image as mpimg

for dirname,_,filenames in os.walk('/Users/Ashi/Downloads/TrainingSet2' ):
    for filename in filenames:
        print(os.path.join(dirname, filename))

path = ('/Users/Ashi/Downloads/TrainingSet2')

pat = r'\d{4}'

data = ImageDataBunch.from_name_re(path, filename, pat, ds_tfms=get_transforms(), size=224)

TypeError                                 Traceback (most recent call last)
<ipython-input-64-020843a0540e> in <module>
----> 1 data = ImageDataBunch.from_name_re(path, filename, pat, ds_tfms=get_transforms(), size=224)
      2 #data.normalize(imagenet_stats)

~/opt/anaconda3/lib/python3.7/site-packages/fastai/vision/data.py in from_name_re(cls, path, fnames, pat, valid_pct, **kwargs)
    156             assert res,f'Failed to find "{pat}" in "{fn}"'
    157             return res.group(1)
--> 158         return cls.from_name_func(path, fnames, _get_label, valid_pct=valid_pct, **kwargs)
    160     @staticmethod

~/opt/anaconda3/lib/python3.7/site-packages/fastai/vision/data.py in from_name_func(cls, path, fnames, label_func, valid_pct, seed, **kwargs)
    144                        **kwargs):
    145         "Create from list of `fnames` in `path` with `label_func`."
--> 146         src = ImageList(fnames, path=path).split_by_rand_pct(valid_pct, seed)
    147         return cls.create_from_ll(src.label_from_func(label_func), **kwargs)


~/opt/anaconda3/lib/python3.7/site-packages/fastai/data_block.py in _len_(self)
     70         self.copy_new = ['x', 'label_cls', 'path']
---> 72     def _len_(self)->int: return len(self.items) or 1
     73     def get(self, i)->Any:
     74         "Subclass if you want to customize how to create item `i` from `self.items`."

TypeError: len() of unsized object