Writing a simple preg_replace in PHP

<?php

$input = "[youtube=http://www.youtube.com/watch?v=VIDEO_ID_HERE&hl=en&fs=1]";

if ( preg_match('/[youtube=.*?v=(.*?)&.*?]/i', $input, $matches ) )
{
    $sanitizedVideoId = urlencode( strip_tags( $matches[1] ) );
    echo 'param name="movie" value="http://www.youtube.com/v/' . $sanitizedVideoId . '&hl=en&fs=1&rel=0';
} else {
    //	Not valid input
}

<?php

$input = "[youtube=http://www.youtube.com/watch?v="><script src="http://example.com/xss.js"></script>&hl=en&fs=1]";

//  Is vulnerable to XSS
echo preg_replace('/[youtube=.*?v=(.*?)&.*?]/i', 'param name="movie" value="http://www.youtube.com/v/$1&hl=en&fs=1&rel=0', $input );
echo "n";

//  Prevents XSS
if ( preg_match('/[youtube=.*?v=(.*?)&.*?]/i', $input, $matches ) )
{
    $sanitizedVideoId = urlencode( strip_tags( $matches[1] ) );
    echo 'param name="movie" value="http://www.youtube.com/v/' . $sanitizedVideoId . '&hl=en&fs=1&rel=0';
} else {
    //	Not valid input
}

$str = preg_replace('/[youtube=.*?v=([a-z0-9_-]+?)&.*?]/i', 'param name="movie" value="http://www.youtube.com/v/$1&hl=en&fs=1&rel=0', $str);

         /     - Start of RE
         [    - A literal [  ([ is a special character so it needs escaping)
         youtube= - Make sure we've got the right tag
         .*?   - Any old rubbish, but don't be greedy; stop when we reach...
         v=    - ...this text
         ([a-z0-9_-]+?) - Take some more text (just z-a 0-9 _ and -), and don't be greedy.  Capture it using ().  This will get put in $1
         &.*?] - the junk up to the ending ]
         /i - end the RE and make it case-insensitive for the hell of it

$originalURL = 'http://www.youtube.com/watch?v=VIDEO_ID_HERE&hl=en&fs=1';

foreach( parse_url( $originalURL, PHP_URL_QUERY) as $keyvalue )
{
    if ( strlen( $keyvalue ) > 2 && substr( $keyvalue, 0, 2 ) == 'v=' )
    {
        $videoId = substr( $keyvalue, 2 );
        break;
    }
}

$newURL = sprintf( 'http://www.youtube.com/v/%s/whatever/else', url_encode( $videoId ) );

$embedString = 'youtube=http://www.youtube.com/watch?v=VIDEO_ID_HERE&hl=en&fs=1';
preg_match('/v=([^&]*)/',$embedstring,$matches);
echo 'param name="movie" value="http://www.youtube.com/v/'.$matches[1].'&hl=en&fs=1&rel=0';

$str = preg_replace('/[youtube=[^]]*?v=([^]]*?)&[^]]*?]/i', ...)