php -r 'echo next(explode( "v=", $argv[1] ) );' https://youtube.com/watch?v=8tRUckf0WbY

php -r 'echo preg_replace ( "/.*v\=([a-z0-9]+).*/i", "$1", $argv[1] );' https://youtube.com/watch?v=8tRUckf0WbY