## 1.截取某字符串的前10个字符串

let sub1 = str.prefix(10)

## 2.截取某字符串的后10个字符串

let str1 = str.suffix(10)

也可以换种写法

let index2 = str.index(str.endIndex, offsetBy: -10)

let sub4 = str[index2..

## 3.截取某字符串的第3个字符到第6个字符范围的字符串

let index3 = str.index(str.startIndex, offsetBy:3)

let index4 = str.index(str.startIndex, offsetBy:6)

let sub4 = str[index3..

## 4.截取指定字符串

截取 access_token开始以后的字符串

```

        if let range = urlStr.range(of:"access_token") {

            query =String(urlStr.suffix(from: range.lowerBound))

        }

```

截取不包含 access_token开始以前的字符串

        if let range = urlStr.range(of:"access_token") {

            query =String(urlStr.suffix(from: range.lowerBound))

        }

  let testStr = "hello world"

    //这种方法和swift3.2 类似

let index1 = testStr.index(testStr.endIndex, offsetBy: -5)

let test1 = String(testStr.suffix(from: index1))

//test1 = "hello"

let index2 = testStr.index(testStr.startIndex, offsetBy: 5)

let test2 = String(testStr.prefix(upTo: index2))//

//test1 = "world"

//当然你也可以这样，是不是更简洁了

let test3 = String(testStr.suffix(5))

//test3 = "hello"

let test4 = String(testStr.prefix(5))