<?php

    $url = $_GET['url'];

    $rules = array(array('/article\/([0-9])/', '/articles/show/$1'));

    $i = 0;
        $found = false;
    //We check if we haven't found any rule that matches to what we want and that we haven't went beyond the array index
        while(!$found && !empty($rules) && $i < count($rules))
          {
             if(preg_match($rules[$i][0], $url))
                {
                  $url = preg_replace($rules[$i][0], $rules[$i][1], $url);
                   $found == true;
                   }
                $i++;
           }

    //....Do what you want with the url