Also here is a solution for you that does not use
Regex
:
string mainString = "<iframe width=\"640\" height=\"360\" src=\"https://www.youtube.com/embed/KVlXccl-XBA\" frameborder=\"0\" allowfullscreen></iframe>";
int start = mainString.IndexOf("src=\"") + 5;
int count = mainString.Length - mainString.IndexOf("\"", start) - 2;
string subString = mainString.Substring(start, count);