<?php 
$content = "{youtube}omg{/youtube}";
$find = array();
$replace = array();

$find[] = '{youtube((?!}).)*}';
$replace[] = '[embed]http://w...content-available-to-author-only...e.com/watch?v=';
$find[] = '{/youtube((?!}).)*}';
$replace[] = '[/embed]';

foreach ( $find as $key => $value ) {
    $find[$key] = '/' . preg_quote($value) . '/';
}
echo preg_replace($find, $replace, $content);