Filter a string by a CharacterSet

StringFiltering.swift

// known-good: Xcode 8.1, Swift 3

extension String {
  func keepingCharacters(inCharacterSet allowedCharacters:CharacterSet) -> String {
    var filteredString:String = ""
    var rangeToCheck:Range<String.Index> = self.startIndex..<self.endIndex
    
    while true {
      if let allowedCharRange = self.rangeOfCharacter(from: allowedCharacters,
                                                   options: [],
                                                   range: rangeToCheck) {
        filteredString.append(self.substring(with: allowedCharRange))
        
        rangeToCheck = Range(uncheckedBounds:
          (lower: allowedCharRange.upperBound, upper: rangeToCheck.upperBound))
      }
      else {
        break
      }
    }
    
    return filteredString
  }
}

extension String {
  func removingCharacters(inCharacterSet forbiddenCharacters:CharacterSet) -> String {
    var filteredString = self
    while true {
      if let forbiddenCharRange = filteredString.rangeOfCharacter(from: forbiddenCharacters)  {
        filteredString.removeSubrange(forbiddenCharRange)
      }
      else {
        break
      }
    }
    return filteredString
  }
}