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