I tend to avoid putting ?, +, *, etc on captures. I don't know if the behaviour is even documented, but the behaviour is as follows: 

(...)?      Returns undef if it matched 0 items.
((?:...)?)  Returns an empty but defined string if it matched 0 items.

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You can use defined to test in the first case.

In reply to Re: Special Variables in Regular Expression by ikegami
in thread Special Variables in Regular Expression by fiat

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