diff --git a/activitypub.php b/activitypub.php
index a483425..6c77a3b 100644
--- a/activitypub.php
+++ b/activitypub.php
@@ -22,7 +22,7 @@ function init() {
 	\defined( 'ACTIVITYPUB_EXCERPT_LENGTH' ) || \define( 'ACTIVITYPUB_EXCERPT_LENGTH', 400 );
 	\defined( 'ACTIVITYPUB_MAX_IMAGE_ATTACHMENTS' ) || \define( 'ACTIVITYPUB_MAX_IMAGE_ATTACHMENTS', 3 );
 	\defined( 'ACTIVITYPUB_HASHTAGS_REGEXP' ) || \define( 'ACTIVITYPUB_HASHTAGS_REGEXP', '(?:(?<=\s)|(?<=<p>)|(?<=<br>)|^)#([A-Za-z0-9_]+)(?:(?=\s|[[:punct:]]|$))' );
-	\defined( 'ACTIVITYPUB_USERNAME_REGEXP' ) || \define( 'ACTIVITYPUB_USERNAME_REGEXP', '(?:[A-Za-z0-9_-]+@((?:[A-Za-z0-9_-]+\.)+[A-Za-z]+))' );
+	\defined( 'ACTIVITYPUB_USERNAME_REGEXP' ) || \define( 'ACTIVITYPUB_USERNAME_REGEXP', '(?:([A-Za-z0-9_-]+)@((?:[A-Za-z0-9_-]+\.)+[A-Za-z]+))' );
 	\defined( 'ACTIVITYPUB_ALLOWED_HTML' ) || \define( 'ACTIVITYPUB_ALLOWED_HTML', '<strong><a><p><ul><ol><li><code><blockquote><pre><img>' );
 	\defined( 'ACTIVITYPUB_CUSTOM_POST_CONTENT' ) || \define( 'ACTIVITYPUB_CUSTOM_POST_CONTENT', "<p><strong>[ap_title]</strong></p>\n\n[ap_content]\n\n<p>[ap_hashtags]</p>\n\n<p>[ap_shortlink]</p>" );
 	\define( 'ACTIVITYPUB_PLUGIN_DIR', plugin_dir_path( __FILE__ ) );
@@ -155,3 +155,41 @@ add_action(
 		$friends_feed->register_parser( Friends_Feed_Parser_ActivityPub::SLUG, new Friends_Feed_Parser_ActivityPub( $friends_feed ) );
 	}
 );
+
+/**
+ * Disable webfinger for known example domains.
+*/
+
+add_filter(
+	'pre_get_remote_metadata_by_actor',
+	function( $metadata, $actor ) {
+		if ( ! $metadata ) {
+			$username = null;
+			$domain = null;
+			if ( preg_match( '/^@?' . ACTIVITYPUB_USERNAME_REGEXP . '$/i', $actor, $m ) ) {
+				$username = $m[1];
+				$domain = $m[2];
+			} else {
+				$p = parse_url( $actor );
+				if ( $p ) {
+					if ( isset( $p['host'] ) ) {
+						$domain = $p['host'];
+					}
+					if ( isset( $p['path'] ) ) {
+						$path_parts = explode( '/', trim( $p['path'], '/' ) );
+						$username = array_pop( $path_parts );
+					}
+				}
+			}
+			if ( strtok( $domain, '.' ) === 'example' ) {
+				$metadata = array(
+					'url' => sprintf( 'https://%s/users/%s/', $domain, $username ),
+					'name' => $username,
+				);
+			}
+		}
+		return $metadata;
+	},
+	10,
+	2
+);
diff --git a/includes/class-mention.php b/includes/class-mention.php
index ddae996..4e305a8 100644
--- a/includes/class-mention.php
+++ b/includes/class-mention.php
@@ -36,7 +36,7 @@ class Mention {
 	 */
 	public static function replace_with_links( $result ) {
 		$metadata = \ActivityPub\get_remote_metadata_by_actor( $result[0] );
-		if ( ! is_wp_error( $metadata ) ) {
+		if ( ! is_wp_error( $metadata ) && ! empty( $metadata['url'] ) ) {
 			$username = ltrim( $result[0], '@' );
 			if ( ! empty( $metadata['name'] ) ) {
 				$username = $metadata['name'];
diff --git a/includes/class-webfinger.php b/includes/class-webfinger.php
index f83214c..ab33411 100644
--- a/includes/class-webfinger.php
+++ b/includes/class-webfinger.php
@@ -38,7 +38,7 @@ class Webfinger {
 			return $link;
 		}
 
-		$url = \add_query_arg( 'resource', 'acct:' . ltrim( $account, '@' ), 'https://' . $m[1] . '/.well-known/webfinger' );
+		$url = \add_query_arg( 'resource', 'acct:' . ltrim( $account, '@' ), 'https://' . $m[2] . '/.well-known/webfinger' );
 		if ( ! \wp_http_validate_url( $url ) ) {
 			$response = new \WP_Error( 'invalid_webfinger_url', null, $url );
 			\set_transient( $transient_key, $response, HOUR_IN_SECONDS ); // Cache the error for a shorter period.
diff --git a/tests/test-functions.php b/tests/test-functions.php
index 68140e0..682d0aa 100644
--- a/tests/test-functions.php
+++ b/tests/test-functions.php
@@ -1,9 +1,28 @@
 <?php
 class Test_Functions extends ActivityPub_TestCase_Cache_HTTP {
+	public function invalid_http_response() {
+		return $this->assertTrue( false ); // should not be called.
+	}
+
 	public function test_get_remote_metadata_by_actor() {
 		$metadata = \ActivityPub\get_remote_metadata_by_actor( 'pfefferle@notiz.blog' );
 		$this->assertEquals( 'https://notiz.blog/author/matthias-pfefferle/', $metadata['url'] );
 		$this->assertEquals( 'pfefferle', $metadata['preferredUsername'] );
 		$this->assertEquals( 'Matthias Pfefferle', $metadata['name'] );
+
+		add_filter( 'pre_http_request', array( $this, 'invalid_http_response' ), 8, 3 );
+		foreach ( array( 'user', 'test' ) as $username ) {
+			foreach ( array( 'example.org', 'example.net' ) as $domain ) {
+				foreach ( array( '@', '' ) as $leading_at ) {
+					$metadata = \ActivityPub\get_remote_metadata_by_actor( $username . '@' . $domain );
+					$this->assertEquals( sprintf( 'https://%s/users/%s/', $domain, $username ), $metadata['url'], $username . '@' . $domain );
+					$this->assertEquals( $username, $metadata['name'], $username . '@' . $domain );
+				}
+				$metadata = \ActivityPub\get_remote_metadata_by_actor( sprintf( 'https://%s/users/%s/', $domain, $username ) );
+				$this->assertEquals( sprintf( 'https://%s/users/%s/', $domain, $username ), $metadata['url'], $username . '@' . $domain );
+				$this->assertEquals( $username, $metadata['name'], $username . '@' . $domain );
+			}
+		}
+		remove_filter( 'pre_http_request', array( $this, 'invalid_http_response' ), 8 );
 	}
 }