add_filter( 'the_content', function( $content ) {
  $content = preg_replace(
    '/^<a.*?title="(https:\/\/twitter.com\/.*?\/status\/\d+)" .*?<\/a>/im',
    '<blockquote class="twitter-tweet"><a href="$1">Tweetを表示</a></blockquote><script async src="https://platform.twitter.com/widgets.js"></script>',
    $content
  );

  return $content;
});