6 invocations of hook_filter()

check_markup in drupal-6.x/modules/filter/filter.module: Run all the enabled filters on a piece of text.
filter_admin_configure in drupal-6.x/modules/filter/filter.admin.inc: Build a form to change the settings for a format's filters.
filter_admin_format_form in drupal-6.x/modules/filter/filter.admin.inc: Generate a filter format form.
filter_admin_format_form_submit in drupal-6.x/modules/filter/filter.admin.inc: Process filter format form submissions.
filter_list_all in drupal-6.x/modules/filter/filter.module: Build a list of all filters.
filter_list_format in drupal-6.x/modules/filter/filter.module: Retrieve a list of filters for a certain format.