The Free On-line Dictionary of Computing (30 December 2018):

??

    A Perl quote-like operator used to delimit a
   regular expression (RE) like "?FOO?" that matches FOO at
   most once.  The normal "/FOO/" form of regular expression will
   match FOO any number of times.  The "??" operator will match
   again after a call to the "reset" operator.

   The operator is usually referred to as "??" but, taken
   literally, an empty RE like this (or "//") actually means to
   re-use the last successfully matched regular expression or, if
   there was none, empty string (which will always match).

   Unix manual page: perlop(1).

   (2009-05-28)