A group() expression returns one or more subgroups of the match.
>>> import re
>>> m = re.match(r'(\w+)@(\w+)\.(\w+)','username@hackerrank.com')
>>> m.group(0) # The entire match
'username@hackerrank.com'
>>> m.group(1) # The first parenthesized subgroup.
'username'
>>> m.group(2) # The second parenthesized subgroup.
'hackerrank'
>>> m.group(3) # The third parenthesized subgroup.
'com'
>>> m.group(1,2,3) # Multiple arguments give us a tuple.
('username', 'hackerrank', 'com')