Does regexp treat metacharacters and literal characters differently when using empty lookaround operators?

    Command: regexp('2015-02-11','(?<=\d+\-)02','match','once')
    ans = '02'

    Command: regexp('2015-02-11','(?<=\d+\-)\d+','match','once')
    ans = '02'

    Command: regexp('2015-02-11','(?<=)2015','match','once')
    ans = '2015'

    Command: regexp('2015-02-11','(?<=)\d+','match','once')
    ans = ''

The empty lookaround operator works when followed by a literal character match. And the \d metacharacter works when following a lookaround operator that isn't empty. Why then does the metacharacter fail when following an empty lookaround operator?