function bbp_make_mentions_clickable_callback( $matches = array() ) {

	// Get user; bail if not found
	$user = get_user_by( 'slug', $matches[1] );
	if ( empty( $user ) || bbp_is_user_inactive( $user->ID ) ) {
		return $matches[0];
	}

	// Filter classes
	$classes = (array) apply_filters( 'bbp_make_mentions_clickable_classes', array(
		'bbp-user-id-' . $user->ID,
		'bbp-user-mention'
	) );

	// Escape & implode if not empty, otherwise an empty string
	$class_str = ! empty( $classes )
		? implode( ' ', array_map( 'sanitize_html_class', $classes ) )
		: '';

	// Create the link to the user's profile
	$url    = bbp_get_user_profile_url( $user->ID );
	$clicky = '<a href="%1$s" class="' . esc_attr( $class_str ) . '">%2$s</a>';
	$anchor = sprintf( $clicky, esc_url( $url ), esc_html( $matches[0] ) );
	$link   = bbp_rel_nofollow( $anchor );

	return $link;
}