oEmbed in WordPress Comments

The code below allows using oEmbed in WordPress comments.

<?php

/**
 * Plugin Name: oEmbed in Comments
 * Description: Allow oEmbeds in comment text. A fork of http://wordpress.org/plugins/oembed-in-comments/
 * Version: 1.2
 * Author: Evan Solomon, modified by Shea Bunge
 */

class oEmbed_Comments {

  /**
	 * Setup filter with correct priority to do oEmbed in comments
	 * @since  1.0
	 * @uses   is_admin()   To make sure we don't do anything in the admin
	 * @uses   add_filter   To register a filter hook
	 * @uses   has_filter() To check if a filter is registered
	 * @return void
	 */
	static function add_filter() {
		if ( is_admin() )
			return;

		/* make_clickable breaks oEmbed regex, make sure we go earlier */
		$clickable = has_filter( 'comment_text', 'make_clickable' );
		$priority = ( $clickable ) ? $clickable - 1 : 10;

		add_filter( 'comment_text', array( __CLASS__, 'oembed_filter' ), $priority );
	}

	/**
	 * Safely add oEmbed media to a comment
	 * @since  1.0
	 * @param  string $comment_text The current comment test
	 * @return string               The modified comment text
	 */
	static function oembed_filter( $comment_text ) {
		global $wp_embed;

		/* Automatic discovery would be a security risk, safety first */
		add_filter( 'embed_oembed_discover', '__return_false', 999 );
		$comment_text = $wp_embed->autoembed( $comment_text );

		/* ...but don't break your posts if you use it */
		remove_filter( 'embed_oembed_discover', '__return_false', 999 );

		return $comment_text;
	}

}

add_action( 'init', array( 'oEmbed_Comments', 'add_filter' ) );

Correction for AJAX functionality:

<?php

/**
 * Class oEmbed_Comments
 * @package RusDTP
 *
 * @see     https://gist.github.com/sheabunge/6018753
 */
class oEmbed_Comments {

	public function __construct() {
		add_action( 'init', [ $this, 'add_filter' ] );
	}

	/**
	 * Setup filter with correct priority to enable oEmbed in comments
	 */
	public function add_filter() {
		global $pagenow;

		$access_page = [ 'index.php', 'admin-ajax.php' ];

		if ( ! in_array( $pagenow, $access_page ) ) {
			return;
		}

		/* make_clickable breaks oEmbed regex, make sure we go earlier */
		remove_filter( 'comment_text', 'make_clickable', 9 );
		add_filter( 'comment_text', 'make_clickable', 12 );

		add_filter( 'comment_text', [ $this, 'oembed_filter' ], 11 );
	}

	/**
	 * Safely add oEmbed media to a comment
	 *
	 * @param string $comment_text The current comment text
	 *
	 * @return string               The modified comment text
	 * @since  1.0
	 */
	public function oembed_filter( $comment_text ) {
		global $wp_embed;

		/* Automatic discovery would be a security risk, safety first */
		add_filter( 'embed_oembed_discover', '__return_false', 999 );
		$comment_text = $wp_embed->autoembed( $comment_text );

		/* ...but don't break your posts if you use it */
		remove_filter( 'embed_oembed_discover', '__return_false', 999 );

		return $comment_text;
	}

}

new oEmbed_Comments();