Use a PCRE to identify valid entity IDs

Toward #90 and #72.

business.php

@@ -25,6 +25,11 @@ function get_content($url)
 
 }
 
+/*
+ * Define the PCRE to match all entity IDs
+ */
+$entity_id_pcre = '/(F|S|T|L|M|[0-9])([0-9]{6})/';
+
 /*
  * If no business ID has been passed in the URL
  */
@@ -37,7 +42,7 @@ function get_content($url)
 /*
  * If the business ID has an invalid format
  */
-elseif ( strlen($_GET['id']) < 7 || strlen($_GET['id']) > 9 )
+elseif ( preg_match($entity_id_pcre, $_GET['id'] == 0) )
 {
     header($_SERVER["SERVER_PROTOCOL"]." 404 Not Found", true, 404);
     exit();