Index: modules/filter/filter.api.php
===================================================================
RCS file: /cvs/drupal/drupal/modules/filter/filter.api.php,v
retrieving revision 1.21
diff -u -p -r1.21 filter.api.php
--- modules/filter/filter.api.php	18 Sep 2010 02:18:35 -0000	1.21
+++ modules/filter/filter.api.php	25 Oct 2010 22:20:41 -0000
@@ -185,6 +185,52 @@ function hook_filter_info() {
 }
 
 /**
+ * Perform alterations on text formats definitions.
+ *
+ * @param $formats
+ *   Array of of all existent text formats.
+ *
+ * @see filter_formats()
+ */
+function hook_filter_formats_alter(&$formats) {
+  $filters['custom_format'] = array(
+    'title' => t('Custom text format'),
+    'description' => t('Allows you to restrict the HTML tags the user can use.'),
+    'process callback' => '_custom_callback',
+    'settings callback' => '_custom_settings_callback',
+    'default settings' => array(
+      'allowed_html' => '<a> <em> <strong> <cite> <blockquote> <code> <ul> <ol> <li> <dl> <dt> <dd>',
+      'filter_html_help' => 1,
+      'filter_html_nofollow' => 0,
+    ),
+    'tips callback' => '_custom_text_format_tips',
+  );
+}
+
+/**
+ * Perform alterations on text formats definitions.
+ *
+ * @param $formats
+ *   Array of of all existent text formats.
+ *
+ * @see filter_list_format()
+ */
+function hook_filter_list_format_alter(&$formats) {
+  $custom_filter = new StdClass;
+  $custom_filter->format = 'custom_format';
+  $custom_filter->module = 'custom_module';
+  $custom_filter->name = 'custom_filter';
+  $custom_filter->weight = 0;
+  $custom_filter->status = 1;
+  $settings = array(
+    'allowed_html' => '<a> <em> <strong>',
+  );
+  $custom_filter->settings = serialize($settings);
+
+  $formats['custom_format']['custom_filter'] = $custom_filter;
+}
+
+/**
  * Perform alterations on filter definitions.
  *
  * @param $info
Index: modules/filter/filter.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/filter/filter.module,v
retrieving revision 1.355
diff -u -p -r1.355 filter.module
--- modules/filter/filter.module	22 Oct 2010 16:36:14 -0000	1.355
+++ modules/filter/filter.module	25 Oct 2010 22:20:43 -0000
@@ -306,7 +306,17 @@ function filter_format_disable($format) 
  * @see filter_format_load()
  */
 function filter_format_exists($format_id) {
-  return (bool) db_query_range('SELECT 1 FROM {filter_format} WHERE format = :format', 0, 1, array(':format' => $format_id))->fetchField();
+  $formats = db_select('filter_format', 'ff')
+    ->fields('ff')
+    ->condition('format', $format_id)
+    ->execute()
+    ->fetchAllAssoc('format');
+
+  // Allow modules to alter the list of formats; e.g., to load formats
+  // exported into code.
+  drupal_alter('filter_formats', $formats);
+
+  return isset($formats[$format_id]);
 }
 
 /**
@@ -415,6 +425,10 @@ function filter_formats($account = NULL)
         ->execute()
         ->fetchAllAssoc('format');
 
+      // Allow modules to alter the list of formats; e.g., to load formats
+      // exported into code.
+      drupal_alter('filter_formats', $formats['all']);
+
       cache_set("filter_formats:{$language->language}", $formats['all']);
     }
   }
@@ -673,6 +687,10 @@ function filter_list_format($format_id) 
       foreach ($result as $record) {
         $filters['all'][$record->format][$record->name] = $record;
       }
+      // Allow modules to alter the filters of all formats; e.g., to complete
+      // formats exported into code.
+      drupal_alter('filter_list_format', $filters['all']);
+
       cache_set('filter_list_format', $filters['all']);
     }
   }