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extension String {
func substring(from: Int?, to: Int?) -> String {
    if let start = from {
        guard start < self.characters.count else {
            return ""
        }
    }

    if let end = to {
        guard end >= 0 else {
            return ""
        }
    }

    if let start = from, let end = to {
        guard end - start >= 0 else {
            return ""
        }
    }

    let startIndex: String.Index
    if let start = from, start >= 0 {
        startIndex = self.index(self.startIndex, offsetBy: start)
    } else {
        startIndex = self.startIndex
    }

    let endIndex: String.Index
    if let end = to, end >= 0, end < self.characters.count {
        endIndex = self.index(self.startIndex, offsetBy: end + 1)
    } else {
        endIndex = self.endIndex
    }

    return self[startIndex ..< endIndex]
}

func substring(from: Int) -> String {
    return self.substring(from: from, to: nil)
}

func substring(to: Int) -> String {
    return self.substring(from: nil, to: to)
}

func substring(from: Int?, length: Int) -> String {
    guard length > 0 else {
        return ""
    }

    let end: Int
    if let start = from, start > 0 {
        end = start + length - 1
    } else {
        end = length - 1
    }

    return self.substring(from: from, to: end)
}

func substring(length: Int, to: Int?) -> String {
    guard let end = to, end > 0, length > 0 else {
        return ""
    }

    let start: Int
    if let end = to, end - length > 0 {
        start = end - length + 1
    } else {
        start = 0
    }

    return self.substring(from: start, to: to)
}
}

And then, you can use:

let string = "Hello,World!"
string.substring(from: 1, to: 7)gets you: ello,Wo

string.substring(to: 7)gets you: Hello,Wo

string.substring(from: 3)gets you: lo,World!

string.substring(from: 1, length: 4)gets you: ello

string.substring(length: 4, to: 7)gets you: o,Wo

Updated substring(from: Int?, length: Int) to support starting from zero.