Validation callback for query tags.
function views_element_validate_tags($element, &$form_state) {
$values = array_map('trim', explode(',', $element['#value']));
foreach ($values as $value) {
if (preg_match("/[^a-z_]/", $value)) {
form_error($element, t('The query tags may only contain lower-case alphabetical characters and underscores.'));
return;
}
}
}