Actually a little bit shorter (and correctly including prefixes) (and,
of course, this is XPath 3.0):

   ('bQc', 'Qc')
    [string-length(substring-before('aabQc', .))
    =
     min((('bQc', 'Qc')!(string-length(substring-before('aabQc', .)))))
     ]


To explain this in English: