Example form is linked to a database table, but won't be saved in this example.
When you submit the form, the value is presented with debug_dump(), so strings are quoted and array values listed.
<?php
	$field = new form_field_postcode($form, 'Postcode');
	$field->db_field_set('postcode');
	$field->format_error_set('Your postcode does not appear to be correct.');
	$field->required_error_set('Your postcode is required.');
?>