import re
r = re.compile(r'(?P<A> a (?P<B> b))', re.X)
m = r.search('ab')
for k in m.groupdict():     
    print k, '=>', m.group(k)    # 'A => ab'             
                                 # 'B => b'
print m.group('A')               # 'ab'
print m.group('B')               # 'b'