s/&((?:(#\d+|0x[\da-f]+)|([a-z0-9]+));?)?/
  $2 ? "&$2;"  # numerical
     : $3 && $isname{$3} ? ($3 eq 'apos' ? "'" : "&$3;")  # named
                         : ($1 ? "&$1" : "&")  # not an entity
/gie;