String+slice.swift

public extension String {
    func slice(from firstString: String, to lastString: String) -> String? {
        return (range(of: firstString)?.upperBound).flatMap { substringFrom in
            (range(of: lastString, range: substringFrom..<endIndex)?.lowerBound).map { substringTo in
                String(self[substringFrom..<substringTo])
            }
        }
    }
}