converting between Camel Case and Snake Case for Swift 4

Camel2SnakeAndSnake2Camel.swift

extension String {
    static private let SNAKECASE_PATTERN:String = "(\\w{0,1})_"
    static private let CAMELCASE_PATTERN:String = "[A-Z][a-z,\\d]*"
    func snake_caseToCamelCase() -> String{
        let buf:NSString = self.capitalized.replacingOccurrences( of: String.SNAKECASE_PATTERN,
                                                      with: "$1",
                                                      options: .regularExpression,
                                                      range: nil) as NSString
        return buf.replacingCharacters(in: NSMakeRange(0,1), with: buf.substring(to: 1).lowercased()) as String
    }
    func camelCaseTosnake_case() throws -> String {
        guard let pattern: NSRegularExpression = try? NSRegularExpression(pattern: String.CAMELCASE_PATTERN,
                                                                 options: []) else {
                                                                    throw NSError(domain: "NSRegularExpression fatal error occured.", code:-1, userInfo: nil)
        }
        
        let input:NSString = (self as NSString).replacingCharacters(in: NSMakeRange(0,1), with: (self as NSString).substring(to: 1).capitalized) as NSString
        var array = [String]()
        let matches = pattern.matches(in: input as String, options:[], range: NSRange(location:0, length: input.length))
        for match in matches {
            for index in 0..<match.numberOfRanges {
                array.append(input.substring(with: match.rangeAt(index)).lowercased())
            }
        }
        return array.joined(separator: "_")
    }
}