[kh99]

This seems to work:

<?php
$pat = '@^http://[\w.]+youtube\.[\w]+/watch[\?\#!]+v=([\w-]+)[\w&;+=-]*[\#t=]*([\d]*)[&;10shdq=]*|^$@';

$strs = array('http://www.youtube.com/watch?v=is_b1ahWN3Q',
                'http://www.youtube.com/watch?&v=is_b1ahWN3Q',
                'http://www.youtube.com/watch#!v=is_b1ahWN3Q',
                'http://www.youtube.com/watch?v=is_b1ahWN3Q&some_string=also_with_special_characters_&;+=-',
                'http://www.youtube.com/watch?v=is_b1ahWN3Q#t=5m',
                'http://www.youtube.com/watch?v=is_b1ahWN3Q#t=5m7s',
                'http://www.youtube.com/watch?v=is_b1ahWN3Q#t=120s',
    );


foreach ($strs as $str)
{
    preg_match($pat, $str, $matches);
    print_r($matches);
    echo("<BR /><BR />\n");
}
?>

and I get this output:

Array ( [0] => http://www.youtube.com/watch?v=is_b1ahWN3Q [1] => is_b1ahWN3Q [2] => ) 

Array ( ) 

Array ( [0] => http://www.youtube.com/watch#!v=is_b1ahWN3Q [1] => is_b1ahWN3Q [2] => ) 

Array ( [0] => http://www.youtube.com/watch?v=is_b1ahWN3Q&some_string=also_with_special_characters_&;+=- [1] => is_b1ahWN3Q [2] => ) 

Array ( [0] => http://www.youtube.com/watch?v=is_b1ahWN3Q#t=5 [1] => is_b1ahWN3Q [2] => 5 ) 

Array ( [0] => http://www.youtube.com/watch?v=is_b1ahWN3Q#t=5 [1] => is_b1ahWN3Q [2] => 5 ) 

Array ( [0] => http://www.youtube.com/watch?v=is_b1ahWN3Q#t=120s [1] => is_b1ahWN3Q [2] => 120 )

It seems the second string doesn't match because the pattern doesn't allow for & where it appears in that string.