How do I get the YouTube video ID from a URL?

You don't need to use a regular expression for this.

var video_id = window.location.search.split('v=')[1];
var ampersandPosition = video_id.indexOf('&');
if(ampersandPosition != -1) {
  video_id = video_id.substring(0, ampersandPosition);
}

I simplified Lasnv's answer a bit.

It also fixes the bug that WebDeb describes.

Here it is:

var regExp = /^.*(youtu\.be\/|v\/|u\/\w\/|embed\/|watch\?v=|\&v=)([^#\&\?]*).*/;
var match = url.match(regExp);
if (match && match[2].length == 11) {
  return match[2];
} else {
  //error
}

Here is a regexer link to play with: http://regexr.com/3dnqv

I made an enhancement to Regex provided by "jeffreypriebe" because he needed a kind of YouTube URL is the URL of the videos when they are looking through a channel.

Well no but this is the function that I have armed.

<script type="text/javascript">
function youtube_parser(url){
    var regExp = /^.*((youtu.be\/)|(v\/)|(\/u\/\w\/)|(embed\/)|(watch\?))\??v?=?([^#&?]*).*/;
    var match = url.match(regExp);
    return (match&&match[7].length==11)? match[7] : false;
}
</script>

These are the types of URLs supported

http://www.youtube.com/watch?v=0zM3nApSvMg&feature=feedrec_grec_index
http://www.youtube.com/user/IngridMichaelsonVEVO#p/a/u/1/QdK8U-VIH_o
http://www.youtube.com/v/0zM3nApSvMg?fs=1&hl=en_US&rel=0
http://www.youtube.com/watch?v=0zM3nApSvMg#t=0m10s
http://www.youtube.com/embed/0zM3nApSvMg?rel=0
http://www.youtube.com/watch?v=0zM3nApSvMg
http://youtu.be/0zM3nApSvMg

Can be found in [http://web.archive.org/web/20160926134334/] http://lasnv.net/foro/839/Javascript_parsear_URL_de_YouTube