bbp_make_mentions_clickable_callback( array $matches = array() )

function bbp_make_mentions_clickable_callback( $matches = array() ) {

	// Get user; bail if not found
	$user = get_user_by( 'slug', $matches[2] );
	if ( empty( $user ) || bbp_is_user_inactive( $user->ID ) ) {
		return $matches[0];
	}

	// Create the link to the user's profile
	$url    = bbp_get_user_profile_url( $user->ID );
	$anchor = '<a href="%1$s" rel="nofollow">@%2$s</a>';
	$link   = sprintf( $anchor, esc_url( $url ), esc_html( $user->user_nicename ) );

	return $matches[1] . $link;
}