用这个方法你可以得到字符串的特定范围。你需要传递起始索引和你想要的字符总数。

extension String{
    func substring(fromIndex : Int,count : Int) -> String{
        let startIndex = self.index(self.startIndex, offsetBy: fromIndex)
        let endIndex = self.index(self.startIndex, offsetBy: fromIndex + count)
        let range = startIndex..<endIndex
        return String(self[range])
    }
}

转换子字符串(Swift 3)到字符串切片(Swift 4)

在Swift 3,4中:

let newStr = str.substring(to: index) // Swift 3
let newStr = String(str[..<index]) // Swift 4

let newStr = str.substring(from: index) // Swift 3
let newStr = String(str[index...]) // Swift 4 

let range = firstIndex..<secondIndex // If you have a range
let newStr = = str.substring(with: range) // Swift 3
let newStr = String(str[range])  // Swift 4

你应该让一侧为空，因此被称为“部分范围”。

let newStr = str[..<index]

同样适用于操作符的部分范围，只需要将另一侧留空即可:

let newStr = str[index...]

请记住，这些范围操作符返回一个Substring。如果你想把它转换成一个字符串，使用string的初始化函数:

let newStr = String(str[..<index])

您可以在这里阅读更多关于新子字符串的信息。

var str = "Hello, playground"
let indexcut = str.firstIndex(of: ",")
print(String(str[..<indexcut!]))
print(String(str[indexcut!...]))

你可以试着用这种方法，并会得到适当的结果。

我写了一个字符串扩展替换' string: subString:'

extension String {
    
    func sliceByCharacter(from: Character, to: Character) -> String? {
        let fromIndex = self.index(self.index(of: from)!, offsetBy: 1)
        let toIndex = self.index(self.index(of: to)!, offsetBy: -1)
        return String(self[fromIndex...toIndex])
    }
    
    func sliceByString(from:String, to:String) -> String? {
        //From - startIndex
        var range = self.range(of: from)
        let subString = String(self[range!.upperBound...])
        
        //To - endIndex
        range = subString.range(of: to)
        return String(subString[..<range!.lowerBound])
    }
    
}

用法:“日期(1511508780012+0530)”sliceByString(from: "("， to: "+") 示例结果:"1511508780012"

PS:可选选项被强制展开。如有需要，请添加类型安全检查。

编程时，我经常用简单的A-Za-z和0-9组成的字符串。不需要困难的索引操作。这个扩展是基于普通的老左/中/右函数。

extension String {

    // LEFT
    // Returns the specified number of chars from the left of the string
    // let str = "Hello"
    // print(str.left(3))         // Hel
    func left(_ to: Int) -> String {
        return "\(self[..<self.index(startIndex, offsetBy: to)])"
    }

    // RIGHT
    // Returns the specified number of chars from the right of the string
    // let str = "Hello"
    // print(str.left(3))         // llo
    func right(_ from: Int) -> String {
        return "\(self[self.index(startIndex, offsetBy: self.length-from)...])"
    }

    // MID
    // Returns the specified number of chars from the startpoint of the string
    // let str = "Hello"
    // print(str.left(2,amount: 2))         // ll
    func mid(_ from: Int, amount: Int) -> String {
        let x = "\(self[self.index(startIndex, offsetBy: from)...])"
        return x.left(amount)
    }
}