从实体中获取config.yml值

我想从symfony2实体的config.yml中获取一个值。

config.yml:

my_example_bundle:
    slug_pattern: "/^[a-z0-9'._'-]{2,20}$/"

实体：

function setUsername($username) {
    $pattern = ""; // need to get it from slug_pattern in config.yml
    if (!preg_match($pattern, $username)) {
        throw new 'InvalidArgumentException("Username has to match " . $pattern);
    }
    ...
}

谢谢！

更新：Dev4TheWeb也发布了一个不错的解决方案：http://dev4theweb.blogspot.ch/2012/08/how-to-access-configuration-values.html