# import necessary libraries
import pandas as pd
import re

# create a sample pandas series
emails = pd.Series(['[email protected]', 'invalid email', \
                    'fake_email.com', '[email protected]'])

# define regex pattern
pattern = r'[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}'

# find valid emails using findall()
result_emails = emails.str.findall(pattern, flags=re.IGNORECASE)

# remove invalid ones from the result
valid_emails = result_emails.apply(lambda x: x[0] if len(x) > 0
                                    else float('NaN'))

# print results
print(valid_emails.dropna())