how to do node preview validation? It is possible to set a Submit function for Preview button in an add/edit node form.

I would like to see if there is a possibility to go one step backward and validate the data to be previewed.

I just and only need to know how to have access to the validation hook for Preview button. (I don't need Submit hook! I only need Validate hook for this button)
There is no difference between Form Submit button's validate hook and Form Preview button's validate hook.

If you set the form's validate hook in HOOK_form_validate, you have set a secondary validation hook (it means if the default validation hook is passed, then it's the turn of the one you have set) which can be used for both Submit button and Preview button:

function MYMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
   $form['#validate'][] = 'XYZ_validate';
}