Source

File: wp-includes/class-wp-customize-widgets.php

public function parse_widget_setting_id( $setting_id ) {
		if ( ! preg_match( '/^(widget_(.+?))(?:\[(\d+)\])?$/', $setting_id, $matches ) ) {
			return new WP_Error( 'widget_setting_invalid_id' );
		}

		$id_base = $matches[2];
		$number  = isset( $matches[3] ) ? intval( $matches[3] ) : null;

		return compact( 'id_base', 'number' );
	}