0*
    (?:
        1\d?
        |
        2 (?:
            [0-4]\d?
            | 
            5[0-5]?
        )
    )
    (?:
        (?:
            # since we got here, the first 1-3 digits formed a number < 256
            # so allow a dot
            \.
            # a near-copy of the surrounding regex goes here to match next octet
        )
        |
        # no dot matched: allow digit-only strings forming numbers up to 2**32-1
        # (and remember we might already have matched up to 3 digits!)
        # this bit needs adjusting to smaller ranges in the nested copies
    )