There are no required external dependencies at all!
$url = $form_values['submitted_tree']['web_site']; $urlpattern = '/^(([\w] :)?
If the message is not a function and not a string it is simply returned as is.
Sometimes it's nice to be able validate field differently depending on the input itself.
So for example adding a constraint of at least 6 characters will be like saying If the attribute is given it must be at least 6 characters.