bp_find_mentions_by_at_sign( array $mentioned_users, string $content )

function bp_find_mentions_by_at_sign( $mentioned_users, $content ) {
	$pattern = '/[@]+([A-Za-z0-9-_\.@]+)\b/';
	preg_match_all( $pattern, $content, $usernames );

	// Make sure there's only one instance of each username.
	$usernames = array_unique( $usernames[1] );

	// Bail if no usernames.
	if ( empty( $usernames ) ) {
		return $mentioned_users;
	}

	// We've found some mentions! Check to see if users exist.
	foreach ( (array) array_values( $usernames ) as $username ) {
		$user_id = bp_get_userid_from_mentionname( $username );

		// The user ID exists, so let's add it to our array.
		if ( ! empty( $user_id ) ) {
			$mentioned_users[ $user_id ] = $username;
		}
	}

	if ( empty( $mentioned_users ) ) {
		return $mentioned_users;
	}

	return $mentioned_users;
}