our $re_bq;
  $re_bq = qr{
    # open tag
    < blockquote
      (?: \s+ style \s* = \s* " .*? " )? \s*
    \s* >
    (?:
      # some character that doesn't start a nested blockquote
      (?!<blockquote\b) .
    |
      # or a whole (recursively) nested blockquote
       (??{ $re_bq })
    )*
    < / blockquote \s* >
  }xsi;
  ...
  $message =~ s{$re_bq\s*\z}{};