The regular expression:

(?-imsx:&[^(amp;)|(lt;)|(gt;)])

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):
----------------------------------------------------------------------
  &                        '&'
----------------------------------------------------------------------
  [^(amp;)|(lt;)|(gt;)     any character except: '(', 'a', 'm', 'p',
  ]                        ';', ')', '|', '(', 'l', 't', ';', ')',
                           '|', '(', 'g', 't', ';', ')'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------