import re


def extract_checksum(room):
    """Given a room identifier of the form:

        'aaa-bbb-cc-d-e-123[abcde]'

    Return the part in square braces:

        'abcde'
    """
    m = re.search(r'\[(?P<checksum>[a-z]{5})\]$', room)
    return m.group('checksum') if m else None