Dictionary.net

A Perl quote-like operator used to delimit aregular expression (RE) like "?FOO?" that matches FOO atmost once. The normal "/FOO/" form of regular expression willmatch FOO any number of times. The "??" operator will matchagain after a call to the "reset" operator.The operator is usually referred to as "??" but, takenliterally, an empty RE like this (or "//") actually means tore-use the last successfully matched regular expression or, ifthere was none, the empty pattern (which will always match).Unix manual page: perlop(1).