php - How strip a section of a url away to get the remaining unique ID -

hi have variable youtube url user-generated. there lot of threads on here regex , not, being executed in js. want run server-side in php, here simple function:

function youtubeid($url){      $url = preg_replace('http://youtu.be/ regex goes here', '', $url);      return $url; } 

markup:

<a href="http://youtu.be/yhxhvmkxnky"><img src="http://img.youtube.com/vi/'.youtubeid($variable).'/0.jpg" /><img src="play.png" alt="watch video" title="watch video"/></a> 

question: know proper regex http://youtu.be/ ?

i'm sure quick answer, i'm not savvy regex. on side note,does know of resources can teach myself regex don't have ask silly syntax questions this? cheers.

why use (relatively slow) regex @ all?

$url = substr($url, strlen('http://youtu.be/')); 

obviously have set start point no need calculate each time call function, doing here demonstrate idea.

edit:

in case there's reason required use regex:

$url = preg_replace('#http\://youtu\.be/#', '', $url); 

cheers