++++------- a single character from this class (an "atom")
    ||||        is required
    vvvv
    [.(]abc
        ^^^
        |||
        +++---- all these characters are required


##</code><code>##


    ++++------- a character from this class is an "atom"
    ||||
    |||| +++--- all these characters are required
    |||| |||
    vvvv vvv
    [.(]?abc
        ^
        |
        +------ 0 or 1 of preceding "atom" is required