The regular expression:

(?-imsx:^(?<!abc)def)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  (?<!                     look behind to see if there is not:
----------------------------------------------------------------------
    abc                      'abc'
----------------------------------------------------------------------
  )                        end of look-behind
----------------------------------------------------------------------
  def                      'def'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------