UIColor from hex

UIColorFromHex.swift

extension UIColor {
    convenience init? (fromHex hex: String) {
        let hexPattern = try! NSRegularExpression(pattern: "^[0-9a-fA-F]{6}$",
                                                  options: [.anchorsMatchLines])
        let range =  NSRange(location: 0, length: hex.characters.count)
        
        guard hexPattern.matches(in: hex, 
                                 options: [],
                                 range: range).count == 1 
        else { return nil }
        
        let positionR = hex.index(hex.startIndex, offsetBy: 2)
        let positionG = hex.index(hex.startIndex, offsetBy: 4)

        guard let r = Double("0x" + hex.substring(to: positionR)),
            let g = Double("0x" + hex.substring(with: positionR..<positionG)),
            let b = Double("0x" + hex.substring(from: positionG)) else { return nil }
        
        self.init(red: CGFloat(r / 255), 
                  green: CGFloat(g / 255), 
                  blue: CGFloat(b / 255), 
                  alpha: 1)
    }
}