using System.Text.RegularExpressions;
public static void Main()
string retval = string.Empty;
string myString = @"<iframe width='560' height='315' src='https://www.youtube.com/embed/DYAbn0AkDOI' frameborder='0' allowfullscreen></iframe>";
Match match = Regex.Match(myString, @"\ssrc='\b(\S*)\b", RegexOptions.IgnorePatternWhitespace | RegexOptions.IgnoreCase);
if (match.Groups.Count > 1)
int pos = match.Value.LastIndexOf("/") + 1;
retval = match.Value.Substring(pos, match.Value.Length - pos);
Console.WriteLine(retval);