pandas ValueError: pattern contains no capture groups

When using regular expression, I get:

import re string = r'http://www.example.com/abc.html' result = re.search('^.*com', string).group() 

In pandas, I write:

df = pd.DataFrame(columns = ['index', 'url']) df.loc[len(df), :] = [1, 'http://www.example.com/abc.html'] df.loc[len(df), :] = [2, 'http://www.hello.com/def.html'] 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 http://www.example.com 1 http://www.hello.com 

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

Or, if you need a Series,

df.url.str.extract(r'(.*.com)', expand=False) 0 http://www.example.com 1 http://www.hello.com 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 http://www.example.com/abc.html http://www.example.com 1 2 http://www.hello.com/def.html http://www.hello.com 

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 http://www.example.com/abc.html www.example.com 1 2 http://www.hello.com/def.html www.hello.com 

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