If you are using YouTubePlayer in your android app and wants to play youtube video on that, so first we need a youtube video url. From that url we can get a video id(11 alphanumeric character set). In below example we are going to generate a video id from a url. By passing a video url we first compile it with a particular expression, if the pattern matches we will get a video id otherwise we get a null value.

This will work on two kinks of youtube video urls either direct or shared url.
Direct url: https://www.youtube.com/watch?v=XXXXXXXXXXX
Shared url: https://youtu.be/XXXXXXXXXXX

private final static String expression = "(?<=watch\\?v=|/videos/|embed\\/|youtu.be\\/|\\/v\\/|\\/e\\/|watch\\?v%3D|watch\\?feature=player_embedded&v=|%2Fvideos%2F|embed%\u200C\u200B2F|youtu.be%2F|%2Fv%2F)[^#\\&\\?\\n]*";

public static String getVideoId(String videoUrl) {
        if (videoUrl == null || videoUrl.trim().length() <= 0){
            return null;
	}

        Pattern pattern = Pattern.compile(expression);
        Matcher matcher = pattern.matcher(videoUrl);

        try {
            if (matcher.find())
                return matcher.group();
        } catch (ArrayIndexOutOfBoundsException ex) {
		ex.printStackTrace();
        }
        return null;
    }