Swift 3.0 String Extension to create NSData from a String representation of a hexadecimal value

gistfile1.txt

extension String {
    
  
        func replace(_ string:String, replacement:String) -> String {
            return self.replacingOccurrences(of: string, with: replacement, options: NSString.CompareOptions.literal, range: nil)
        }
        
        func removeWhitespace() -> String {
            return self.replace(" ", replacement: "")
        }
    
    
    
    /// Create NSData from hexadecimal string representation
    ///
    /// This takes a hexadecimal representation and creates a NSData object. Note, if the string has any spaces, those are removed. Also if the string started with a '<' or ended with a '>', those are removed, too. This does no validation of the string to ensure it's a valid hexadecimal string
    ///
    /// The use of `strtoul` inspired by Martin R at http://stackoverflow.com/a/26284562/1271826
    ///
    /// - returns: NSData represented by this hexadecimal string. Returns nil if string contains characters outside the 0-9 and a-f range.
    
    func dataFromHexadecimalString() -> NSData? {
        //let trimmedString = self.stringByTrimmingCharactersInSet(NSCharacterSet(charactersInString: "<> ")).stringByReplacingOccurrencesOfString(" ", withString: "")
        var trimmedString = self.replace("<", replacement: "")
        trimmedString = self.replace(">", replacement: "")
        trimmedString = self.removeWhitespace()
        
        // make sure the cleaned up string consists solely of hex digits, and that we have even number of them
        
        var error: NSError?
        let regex: NSRegularExpression?
        do {
            regex = try NSRegularExpression(pattern: "^[0-9a-f]*$", options: .caseInsensitive)
        } catch let error1 as NSError {
            error = error1
            regex = nil
        }
        let found = regex?.firstMatch(in: trimmedString, options: [], range: NSMakeRange(0, trimmedString.characters.count))
        if found == nil || found?.range.location == NSNotFound || trimmedString.characters.count % 2 != 0 {
            return nil
        }
        
        // everything ok, so now let's build NSData
        
        let data = NSMutableData(capacity: trimmedString.characters.count / 2)
        
        for i in stride(from:0,to:trimmedString.characters.count,by:2) {
            let start = trimmedString.index(trimmedString.startIndex, offsetBy: i)
            let end = trimmedString.index(trimmedString.startIndex, offsetBy: i+1)
            
            // get a sub-string with a ClosedRange
            let range = start...end
            
            let byteString = trimmedString[range]
            let num=UInt8(byteString.withCString { strtoul($0, nil, 16) })
            print(byteString)
            print(i)
            data?.append([num] as [UInt8], length: 1)
        }
        
        return data
    }

}