Hook into Drupal registration and validate user info against business logic - drupal-6

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 entries to drupal database. I will be checking against my old database.
When a user tries to register in Drupal, I need to check whether the username he gave is already present in that list of 50000 (and growing) entries. If it exists, I need to cancel the registration showing an error msg saying username exists..
Which hook should I use? If my code figures that the validation failed, How can I tell drupal to display an error msg?
Edit: I hooked into the hook_user and checked for the 'validate' op. I am able to validate and assign error messages. But it is happening for all forms. I want to validate only the new account creation form. How can I do that?
Thanks.

Answer 1

You should register an additional validation callback function for the registration form using hook_form_FORM_ID_alter(), somewhat like so:
// Alter the registration form
function yourModuleName_form_user_register_alter(&$form, &$form_state) {
// Add your own function to the array of validation callbacks
$form['#validate'][] = 'yourModuleName_user_register_validate';
}
// Perform your own validation
function yourModuleName_user_register_validate($form, &$form_state) {
// Extract the submitted name
$name = $form_state['values']['name'];
// Check it according to your own logic
$is_valid_name = your_check_for_valid_name();
// File error, when not valid
if (!$is_valid) {
form_set_error('name', t('Name already taken, please choose a different one'));
}
}

Answer 2

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

Answer 3

Here are some examples for Drupal 7:
/**
* Implements of hook_user_insert().
*/
function foo_user_insert(&$edit, $account, $category) {
// foo_user_submit($edit, $account);
}
/**
* Implementation of hook_user_delete().
*/
function foo_user_delete($account) {
// foo_user_delete($account);
}
/**
* Implements hook_form_FORM_ID_alter().
* Form ID: user_register_form
*/
function foo_form_user_register_form_alter($form, &$form_state) {
if ($form['#user_category'] == 'account' && !isset($form['#user']->uid)) {
// Foo code
}
}
/**
* Implements hook_form_FORM_ID_alter().
* Form ID: user_profile_form
*/
function foo_form_user_profile_form_alter($form, &$form_state) {
// Set a custom form validate and submit handlers.
$form['#validate'][] = 'foo_account_validate';
$form['#submit'][] = 'foo_account_submit';
}
/**
* Implements of hook_form_alter().
* This is the same as: hook_form_FORM_ID_alter()
*/
function foo_form_alter(&$form, $form_state, $form_id) {
switch ($form_id) {
case "user_profile_form":
case "user_register_form":
break;
}
}

Answer 4

Consider using the Username Originality AJAX check module: https://www.drupal.org/project/username_check