The example for getting substring of string with a range in Swift 4.

Substrings.swift

import Foundation

var greeting = "Hello, Swift 4."

let startIndexOffsetBy = 7

let endIndexOffsetBy = -1

let startIndex = greeting.index(greeting.startIndex, offsetBy: startIndexOffsetBy)

let endIndex = greeting.index(greeting.endIndex, offsetBy: endIndexOffsetBy)

let rangeOfSubstring = [startIndex...endIndex]

let substring = String(greeting[startIndex...endIndex])

print(substring)