I am trying to modify/sanitize values returned from additional profile fields which I have added using the user_contactmethods filter. I am able to validate and throw errors using the user_profile_update_errors action, but I can't find a way to filter/modify/sanitize values.
Another way of putting it: how do I take a value like Twitter Name, and add a '@' character if there isn't one - I don't need the regex, I am looking for the hook to enable me to run the function against the $_POST['twitter'] before the meta update. The order of things is quite confusing.