using System;
using System.IO;
using System.Text.RegularExpressions;
public class Test
{
	public static void Main()
	{
		var s = "background-image: url('https://s...content-available-to-author-only...s.com/files.domain.com/uploads/image/file/168726/carousel_IMG_6455.jpg')";
		var res = Regex.Match(s, @"'(.*?)'").Groups[1].Value ?? string.Empty;
		Console.WriteLine(res);
	}
}