UserValidationTestCase::testUsernames in core/modules/user/tests/user.test
user_account_form in core/modules/user/user.module
Helper function to add default user account fields to user registration and edit form.
user_login in core/modules/user/user.module
Form builder; the main user login form.
user_login_block in core/modules/user/user.module
user_pass in core/modules/user/user.pages.inc
Form builder; request a password reset.
user_validate_name in core/modules/user/user.module
Verify the syntax of the given name.
_install_configure_form in core/includes/install.core.inc
Form constructor for a site configuration form.