Note: this list is generated by pattern matching, so it may include some functions that are not actually implementations of this hook.

bartik_form_alter in core/themes/bartik/bartik.theme
Implements hook_form_alter() to add classes to the search form.
bartik_theme_suggestions_form_alter in core/themes/bartik/bartik.theme
Implements hook_theme_suggestions_HOOK_alter() for form templates.
block_form_form_test_alter_form_alter in core/modules/system/tests/modules/form_test/form_test.module
Implements hook_form_FORM_ID_alter() on behalf of block.module.
book_form_node_confirm_form_alter in core/modules/book/book.module
Implements hook_form_BASE_FORM_ID_alter() for \Drupal\node\Form\NodeDeleteForm.
book_form_node_form_alter in core/modules/book/book.module
Implements hook_form_BASE_FORM_ID_alter() for \Drupal\node\NodeForm.
comment_form_field_storage_config_edit_form_alter in core/modules/comment/comment.module
Implements hook_form_FORM_ID_alter() for 'field_storage_config_edit_form'.
comment_form_field_ui_display_overview_form_alter in core/modules/comment/comment.module
Implements hook_form_FORM_ID_alter().
comment_form_field_ui_field_storage_add_form_alter in core/modules/comment/comment.module
Implements hook_form_FORM_ID_alter() for field_ui_field_storage_add_form.
comment_form_field_ui_form_display_overview_form_alter in core/modules/comment/comment.module
Implements hook_form_FORM_ID_alter().
config_translation_test_form_config_translation_add_form_alter in core/modules/config_translation/tests/modules/config_translation_test/config_translation_test.module
Implements hook_form_FORM_ID_alter() for ConfigTranslationAddForm.
config_translation_test_form_config_translation_edit_form_alter in core/modules/config_translation/tests/modules/config_translation_test/config_translation_test.module
Implements hook_form_FORM_ID_alter() for ConfigTranslationEditForm.
config_translation_test_form_config_translation_form_alter in core/modules/config_translation/tests/modules/config_translation_test/config_translation_test.module
Implements hook_form_BASE_FORM_ID_alter() for ConfigTranslationFormBase.
contact_form_user_form_alter in core/modules/contact/contact.module
Implements hook_form_FORM_ID_alter().
contact_storage_test_form_contact_form_form_alter in core/modules/contact/tests/modules/contact_storage_test/contact_storage_test.module
Implements hook_form_FORM_ID_alter() for contact_form_form().
content_moderation_form_alter in core/modules/content_moderation/content_moderation.module
Implements hook_form_alter().
content_translation_form_alter in core/modules/content_translation/content_translation.module
Implements hook_form_alter().
content_translation_form_field_config_edit_form_alter in core/modules/content_translation/content_translation.module
Implements hook_form_FORM_ID_alter() for 'field_config_edit_form'.
content_translation_form_language_content_settings_form_alter in core/modules/content_translation/content_translation.module
Implements hook_form_FORM_ID_alter() for language_content_settings_form().
editor_form_filter_format_form_alter in core/modules/editor/editor.module
Implements hook_form_BASE_FORM_ID_alter() for \Drupal\filter\FilterFormatEditForm.
entity_test_form_entity_test_form_alter in core/modules/system/tests/modules/entity_test/entity_test.module
Implements hook_form_BASE_FORM_ID_alter().
entity_test_form_node_form_alter in core/modules/system/tests/modules/entity_test/entity_test.module
Implements hook_form_BASE_FORM_ID_alter().
field_form_config_admin_import_form_alter in core/modules/field/field.module
Implements hook_form_FORM_ID_alter().
field_test_field_widget_form_alter in core/modules/field/tests/modules/field_test/field_test.module
Implements hook_field_widget_form_alter().
field_ui_form_field_ui_field_storage_add_form_alter in core/modules/field_ui/field_ui.module
Implements hook_form_FORM_ID_alter() for 'field_ui_field_storage_add_form'.
field_ui_form_node_type_form_alter in core/modules/field_ui/field_ui.module
Implements hook_form_FORM_ID_alter().
field_ui_test_form_entity_view_display_edit_form_alter in core/modules/field_ui/tests/modules/field_ui_test/field_ui_test.module
Implements hook_form_FORM_BASE_ID_alter().
form_test_form_alter in core/modules/system/tests/modules/form_test/form_test.module
Implements hook_form_alter().
form_test_form_form_test_alter_form_alter in core/modules/system/tests/modules/form_test/form_test.module
Implements hook_form_FORM_ID_alter().
form_test_form_form_test_vertical_tabs_access_form_alter in core/modules/system/tests/modules/form_test/form_test.module
Implements hook_form_FORM_ID_alter() for form_test_vertical_tabs_access_form().
form_test_form_user_register_form_alter in core/modules/system/tests/modules/form_test/form_test.module
Implements hook_form_FORM_ID_alter() for the registration form.
forum_form_node_form_alter in core/modules/forum/forum.module
Implements hook_form_BASE_FORM_ID_alter() for \Drupal\node\NodeForm.
forum_form_taxonomy_term_form_alter in core/modules/forum/forum.module
Implements hook_form_FORM_ID_alter() for \Drupal\taxonomy\TermForm.
forum_form_taxonomy_vocabulary_form_alter in core/modules/forum/forum.module
Implements hook_form_BASE_FORM_ID_alter() for \Drupal\taxonomy\VocabularyForm.
language_form_alter in core/modules/language/language.module
Implements hook_form_alter().
locale_form_language_admin_add_form_alter in core/modules/locale/locale.module
Implements hook_form_FORM_ID_alter() for language_admin_add_form().
locale_form_language_admin_edit_form_alter in core/modules/locale/locale.module
Implements hook_form_FORM_ID_alter() for language_admin_edit_form().
locale_form_language_admin_overview_form_alter in core/modules/locale/locale.module
Implements hook_form_FORM_ID_alter() for language_admin_overview_form().
menu_ui_form_node_form_alter in core/modules/menu_ui/menu_ui.module
Implements hook_form_BASE_FORM_ID_alter() for \Drupal\node\NodeForm.
menu_ui_form_node_type_form_alter in core/modules/menu_ui/menu_ui.module
Implements hook_form_FORM_ID_alter() for \Drupal\node\NodeTypeForm.
node_form_system_themes_admin_form_alter in core/modules/node/node.module
Implements hook_form_FORM_ID_alter().
node_test_form_alter in core/modules/node/tests/modules/node_test/node_test.module
Implements hook_form_alter().
path_form_node_form_alter in core/modules/path/path.module
Implements hook_form_BASE_FORM_ID_alter() for \Drupal\node\NodeForm.
search_form_search_block_form_alter in core/modules/search/search.module
Implements hook_form_FORM_ID_alter() for the search_block_form form.
seven_form_node_form_alter in core/themes/seven/seven.theme
Implements hook_form_BASE_FORM_ID_alter() for \Drupal\node\NodeForm.
standard_form_install_configure_form_alter in core/profiles/standard/standard.profile
Implements hook_form_FORM_ID_alter() for install_configure_form().
system_form_alter in core/modules/system/system.module
Implements hook_form_alter().
system_form_form_test_alter_form_alter in core/modules/system/tests/modules/form_test/form_test.module
Implements hook_form_FORM_ID_alter() on behalf of system.module.
system_form_user_form_alter in core/modules/system/system.module
Implements hook_form_FORM_ID_alter() for \Drupal\user\AccountForm.
system_form_user_register_form_alter in core/modules/system/system.module
Implements hook_form_FORM_ID_alter() for \Drupal\user\RegisterForm.
user_form_test_form_user_cancel_form_alter in core/modules/user/tests/modules/user_form_test/user_form_test.module
Implements hook_form_FORM_ID_alter() for user_cancel_form().
views_form_views_exposed_form_alter in core/modules/views/views.module
Implements hook_form_alter() for the exposed form.
views_test_checkboxes_theme_form_views_exposed_form_alter in core/modules/views/tests/themes/views_test_checkboxes_theme/views_test_checkboxes_theme.theme
Changes an exposed "type" filter from a multi-select to checkboxes.