# the following are comments:
# <!-- ab -- cd -->   <!-- ab -->    <!---->
# <!-- ab -- cd -- >  <!-- ab -- >   <!---- >

my $comment = qr{
  <!--              # <!--
  [^-]*             # 0 or more non -'s
  (?:               
    (?! -- \s* > )    # that's not --, space, then >
    -                 # a -
    [^-]*             # 0 or more non -'s
  )*                # 0 or more times
  -- \s* >          # --, space, then >
}x;