import Foundation

extension String {
    public func tailString(from offset: Int) -> String {
        return offset > -1 ? "" : self.substring(
            from: self.index(self.endIndex,
            offsetBy: abs(offset) > characters.count ? -characters.count : offset)
        )
    }

    public func leadString(until offset: Int) -> String {
        return offset < 0 ? "" : self.substring(
            to: self.index(self.startIndex,
            offsetBy: abs(offset) > characters.count ? characters.count : offset)
        )
    }
}

let maskedNum = "4185xxxxxxxx9024"
let lastDigits = maskedNum.tailString(from: -4)    // output: "9024"
let leadDigits = maskedNum.leadString(until: 4)    // output: "4185"