Custom Validation

Hello,
i just bought this plugin and i need your help. I want to add custom validation to input. I have added custom input field with meta name “pwz” and i have function like this. How to connect it?

		function check_pwz_number ($number)

		{
		    if (strlen($number) != 7 || preg_match('/^0+$/', $number)) return false;
		    $c = (int)$number[0];
		    $n = substr($number, 1);
		    $s = 0;
		    for ($i = 1; $i <= 6; $i ++) $s += (int)$n[$i - 1] * $i;
		    return ($s % 11) == $c ? true : false;
		}

Thanks
K