send REQUEST_URI with header location redirect via settings.php
While it's technically possible to use Drupal contrib to perform the redirect, it's faster and more efficient to redirect without having to fully bootstrap your web application.

For web only actions, like redirects, check for the existence of $_SERVER['PANTHEON_ENVIRONMENT'] - if it exists, it will contain a string with the current environment (dev, test or live).

To perform redirection for the pattern you've supplied, make sure to only match for it at the beginning of the URI in order to avoid an infinite loop problem:

// Pantheon - web only.
if (isset($_SERVER['PANTHEON_ENVIRONMENT'])) {
    // Check if you only want redirection from the live web environment.
    if ($_SERVER['PANTHEON_ENVIRONMENT'] == 'live') {
        if (preg_match('/^\/article\/[0-9]?[0-9]\/[0-9]?[0-9]\/[aA-zZ].+/', $_SERVER['REQUEST_URI'])) {
            header('HTTP/1.0 301 Moved Permanently');
            header('Location: http://answers3x-ec-core-community.gotpantheon.com/catchall/?dest=' . $_SERVER["REQUEST_URI"]);
            exit();
        }
    }
}


Sources: 

Configuring settings.php

PHP preg_match() Examples

preg_match