/**
 * String Expression Operators: https://docs.mongodb.com/manual/reference/operator/aggregation/#string-expression-operators
 */
import { Options } from '../../../core';
/**
 * Returns a substring of a string, starting at a specified index position and including the specified number of characters.
 * The index is zero-based.
 *
 * @param obj
 * @param expr
 * @returns {string}
 */
export declare function $substr(obj: object, expr: any, options: Options): any;