pandas ValueError: pattern contains no capture groups

When using regular expression, I get:

import re
string = r'
result = re.search('^.*com', string).group()

In pandas, I write:

df = pd.DataFrame(columns = ['index', 'url'])
df.loc[len(df), :] = [1, '
df.loc[len(df), :] = [2, '
df.str.extract('^.*com')
ValueError: pattern contains no capture groups

How to solve the problem?

Thanks.

3 Answers

According to the docs, you need to specify a capture group (i.e., parentheses) for str.extract to, well, extract.

Series.str.extract(pat, flags=0, expand=True)
For each subject string in the Series, extract groups from the first match of regular expression pat.

Each capture group constitutes its own column in the output.

df.url.str.extract(r'(.*.com)') 0
0
1 

# If you need named capture groups,
df.url.str.extract(r'(?P<URL>.*.com)') URL
0
1 

Or, if you need a Series,

df.url.str.extract(r'(.*.com)', expand=False)
0
1
Name: url, dtype: object

You need specify column url with () for match groups:

df['new'] = df['url'].str.extract(r'(^.*com)')
print (df) index url new
0 1
1 2 

Try this python library, works well for this purpose:

Using urllib.parse

from urllib.parse import urlparse
df['domain']=df.url.apply(lambda x:urlparse(x).netloc)
print(df) index url domain
0 1
1 2