[SOLVED] Define a callback function in a plugin

I'm writing a little plugin that checks pages for plain text email addresses and turns them into javascript-encrypted mailto links. I have defined a function munge($address) and I'm using it with preg_replace_callback like this:

public function onPageContentRaw(Event $e)
{
    // Get the current raw content
    $content = $e['page']->getRawContent();

    // find plain text email addresses and replace them with munge() results
    $content = preg_replace_callback('/([a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,6})/', 'munge', $content);
}

However, I get a Crikey! error saying "preg_replace_callback(): Requires argument 2, 'munge', to be a valid callback". So how do I define the munge function correctly?

Thanks for your time!

Side note: I do know that this regex will not catch all valid email addresses. This is not a problem since I know when people putting content into Grav sites that I have set up have whack email addresses – if this ever happens I will fix the regex.