Validating Name or Last Name

This script basically prevents numbers from being used in a field (or other non-alphabetic characters). This validation does not ensure name is mandatory, it should be ensured by field settings.

$name = $form_values['submitted_tree']['name'];
if (strlen(trim($name)) > 0 && !ereg('^[a-z A-Z\'\-]+$', $name)) {
  form_set_error('submitted][name', t('Your name contains invalid characters. Only letters are allowed!'));
}