Hook into Drupal registration and validate user info against business logic

Question

I want to hook into the registration module. I already have a database of 50000 users who use my old website. Now I am migrating to Drupal.

I still haven\'t migrated the

Answer 1

Henrik Opel answer work on Drupal 6. For Drupal 7 use yourModuleName_form_user_register_form_alter