Swift String substring

StringHelper.swift

extension String {
    subscript (r: Range<Int>) -> String {
        get {
            let startIndex = self.startIndex.advancedBy(r.startIndex)
            let endIndex = startIndex.advancedBy(r.endIndex - r.startIndex)
            return self[Range(start: startIndex, end: endIndex)]
        }
    }
    func substring(from: Int) -> String {
        let end = self.characters.count
        return self[from..<end]
    }
    func substring(from: Int, length: Int) -> String {
        let end = from + length
        return self[from..<end]
    }
}

var s = "Hello, playground"

s[0...5]
s[0..<5]
s.substring(5)
s.substring(0,length: 3)