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

'ab' =~ /(?<A> a (?<B> b))/x;
print "$_ => $+{$_}\n" for (keys(%+));    # 'A => ab'
                                          # ... nothing, thats it
print "$+{'A'}\n";                        # 'ab'
print "$+{'B'}\n";                        # 'b'