删除dataframe列中的非字母字符串

import pandas as pd
import re

df = pd.DataFrame({'col1': ['abc', '123', 'def', '456'], 'col2': ['xyz', '789', 'ghi', '012']})

df['col1'] = df['col1'].apply(lambda x: re.sub('[^a-zA-Z]', '', x))

print(df)

  col1 col2
0  abc  xyz
1       789
2  def  ghi
3       012