Source

File: wp-includes/class-wp-widget.php

public function get_field_name( $field_name ) {
		$pos = strpos( $field_name, '[' );
		if ( false === $pos ) {
			return 'widget-' . $this->id_base . '[' . $this->number . '][' . $field_name . ']';
		} else {
			return 'widget-' . $this->id_base . '[' . $this->number . '][' . substr_replace( $field_name, '][', $pos, strlen( '[' ) );
		}
	}