string_replacing.txt

// Apply mask to credit card number
func applyMask(number: String) -> String {
        let mask = "%2A%2A%2A+%2A%2A%2A%2A+%2A%2A%2A%2A+"
        let startIndex = number.startIndex
        
        let range = startIndex.advancedBy(1) ..< number.endIndex.advancedBy(-4)
        let substring = number.substringWithRange(range)
        let replacedWithMask = number.stringByReplacingOccurrencesOfString(substring, withString: mask)
        
        return replacedWithMask
}