local our %known = map { $_ => 1 } qw(
   eacute
   Eacute
   ecirc
   Ecirc
   ...
);

s/
   &
   (?!
      (?:
         \# (?: x[\da-f]+ | \d+ )
      |
         ([a-z]+) (?(?{ !$known{$1} }) (?!) )
      )
      ;
   )
/&/xi