set_value() not working on custom validation field

[eluser]Aken[/eluser]
The problem is you aren't returning TRUE in your valid_date() callback if the field passes. This returns the NULL value, which the form validation library assumes as you altering the value rather than validating it (kind of like running trim() - it alters the value).

Do something like this instead:

Code:

public function valid_date($val)
{
    return (bool) preg_match('^(0[1-9]|1[012])[/](0[1-9]|[12][0-9]|3[01])[/](19|20)\d\d$^', $val);
}