If the video IDs are guaranteed to be at the end, you can do a pattern match, which is probably a lot faster:

def get_video_id("http://youtu.be/" <> id), do: id
def get_video_id("http://www.youtube.com/watch?v=" <> id), do: id
def get_video_id("http://www.youtube.com/?v=" <> id), do: id

How you can append that video ID to the string "http://www.youtube.com/embed/" is left as an exercise.

I’d prefer to make it HTTPS ready and solve it via meta programming, roughly like this:

Enum.each(["http", "https"], fn protocol ->
  Enum.each(["youtu.be/", "www.youtube.com/watch?v=", "www.youtube.com/?v="], fn prefix ->
    def get_video_id(unquote(protocol <> "://" <> prefix) <> id), do: id
  end)
end)

But probably the best version is to properly parse the URL and extract the video ID from the parsed URL. Pattern matching as shown by me will not work correctly when there are other parameters in the query string, and regular expressions that cover those possibilities tend to grow into an unreadable and slow monstrosity.