_install_configure_form in includes/install.core.inc

Form constructor for a site configuration form.

user_account_form in modules/user/user.module

Helper function to add default user account fields to user registration and edit form.

user_login in modules/user/user.module

Form builder; the main user login form.

user_login_block in modules/user/user.module

user_pass in modules/user/user.pages.inc

Form builder; request a password reset.

user_validate_name in modules/user/user.module

Verify the syntax of the given name.

UserValidationTestCase::testUsernames in modules/user/tests/user.test