Slugify routine in Swift 4.1

slugify.swift

////
// Slugify
// A String extension that converts a text string into a valid URL
// Derived from https://github.com/paulyoung/Slugify/blob/master/Slugify/Slugify.swift
// By Jeff Algera - jeffalgera.com
////

extension String {
  private static let allowedCharacters = NSCharacterSet(charactersIn: "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-")
  
  public func slugify() -> String {
    let cocoaString = NSMutableString(string: self) as CFMutableString
    CFStringTransform(cocoaString, nil, kCFStringTransformToLatin, false)
    CFStringTransform(cocoaString, nil, kCFStringTransformStripCombiningMarks, false)
    CFStringLowercase(cocoaString, .none)

    return String(cocoaString)
      .components(separatedBy: String.allowedCharacters.inverted)
      .filter { $0 != "" }
      .joined(separator: "-")
  }
}