<color=#FF00FF>kappa</color> -> NSAttributedString

RichTextParser.swift

class RichTextParser {
    class func parse(text: String) throws -> NSAttributedString {
        let colorPattern = "<color=#([\\w]{6})>(.+?)</color>"
        let attributedText = NSMutableAttributedString()

        do {
            let nsText = (text as NSString)
            let regex = try NSRegularExpression(pattern: colorPattern, options: [])

            let matches = regex.matches(in: text, options: [], range: NSRange(location: 0, length: nsText.length))

            var currentIndex = 0

            for (index, match) in matches.enumerated() {
                let range = match.range(at: 0)
                let rgb = nsText.substring(with: match.range(at: 1))
                let text = nsText.substring(with: match.range(at: 2))

                let myRange = NSRange(location: currentIndex, length: range.location - currentIndex)
                let rgbValues = parseRGB(rgb: rgb)

                attributedText.append(NSAttributedString(string: nsText.substring(with: myRange)))
                attributedText.append(NSAttributedString(
                    string: text,
                    attributes: [
                    NSAttributedStringKey.foregroundColor: UIColor(red: rgbValues.red / 255.0, green: rgbValues.green / 255.0, blue: rgbValues.blue / 255.0, alpha: 1.0)
                    ]))

                currentIndex = range.location + range.length

                if index == matches.count - 1 {
                    attributedText.append(
                        NSAttributedString(string: nsText.substring(with: NSRange(location: currentIndex, length: nsText.length - currentIndex))))
                }

            }

            return attributedText

        } catch let e {
            throw e
        }
    }

    private class func parseRGB(rgb: String) -> (red: CGFloat, green: CGFloat, blue: CGFloat) {
        let red = Int(rgb[rgb.index(rgb.startIndex, offsetBy: 0)..<rgb.index(rgb.startIndex, offsetBy: 2)], radix: 16) ?? 0
        let green = Int(rgb[rgb.index(rgb.startIndex, offsetBy: 2)..<rgb.index(rgb.startIndex, offsetBy: 4)], radix: 16) ?? 0
        let blue = Int(rgb[rgb.index(rgb.startIndex, offsetBy: 4)..<rgb.index(rgb.startIndex, offsetBy: 6)], radix: 16) ?? 0
        return (red: CGFloat(red),
                green: CGFloat(green),
                blue: CGFloat(blue))
    }
}

Swift 4 support.