This video shows how to target a specific form with the hook_form_FORM_ID_alter and creating a customized validation function for a form.
Note: There is a typo in this video. (The code is correct in the downloadable example file attached to the previous video.)
In the demo_validate_password()
function, the following line shown in the video if (in_array($form['values']['pass'], $badpasswords)) {
should be if (in_array($form_state['values']['pass'], $badpasswords)) {
.