add_filter('gform_validation_1', 'custom_validation');
function custom_validation($validation_result) {

// set the form validation to false
$form = $validation_result["form"];

$vin = trim($_POST['input_7']);

	// setup array of letter values
	$value['A'] = 1;
	$value['B'] = 2;
	$value['C'] = 3;
	$value['D'] = 4;
	$value['E'] = 5;
	$value['F'] = 6;
	$value['G'] = 7;
	$value['H'] = 8;
	$value['J'] = 1;
	$value['K'] = 2;
	$value['L'] = 3;
	$value['M'] = 4;
	$value['N'] = 5;
	$value['P'] = 7;
	$value['R'] = 9;
	$value['S'] = 2;
	$value['T'] = 3;
	$value['U'] = 4;
	$value['V'] = 5;
	$value['W'] = 6;
	$value['X'] = 7;
	$value['Y'] = 8;
	$value['Z'] = 9;

	// setup digit weights
	$weight[0] = 8; // 1st position
	$weight[1] = 7;
	$weight[2] = 6;
	$weight[3] = 5;
	$weight[4] = 4;
	$weight[5] = 3;
	$weight[6] = 2;
	$weight[7] = 10;
	$weight[8] = 0; // 9th position, this is the check digit
	$weight[9] = 9;
	$weight[10] = 8;
	$weight[11] = 7;
	$weight[12] = 6;
	$weight[13] = 5;
	$weight[14] = 4;
	$weight[15] = 3;
	$weight[16] = 2; // 17th position

	$char = str_split($vin); // split string into character array
	$total = 0;

	// loop though each character of the vin
	for ($i = 0; $i < 17; $i++) {
		if (is_numeric($char[$i])) {
			// use number
			// update total
			$total = $total + ($char[$i] * $weight[$i]);
		} elseif (array_key_exists($char[$i], $value)) {
			// use value of letter
			// update total
			$total = $total + ($value[$char[$i]] * $weight[$i]);
		} else {
			// error illegal character used
			$validation_result["is_valid"] = false;
			$form["fields"][0]["failed_validation"] = true;
			$form["fields"][0]["validation_message"] = "Please make sure your VIN contains only letters and numbers and letters are all uppercase";
		}
	}

	$mod = $total % 11; // find remainder after dividing by 11

	// if mod is 10 set the check_digit to X
	if ($mod == 10) {
		$checkDigit = 'X';
	} else {
		$checkDigit = $mod;
	}

	// check if the 9th character in the string (the check digit) equals the calculated value
	if ($char[8] == $checkDigit) {
	} else {
		$validation_result["is_valid"] = false;
		$form["fields"][0]["failed_validation"] = true;
		$form["fields"][0]["validation_message"] = "The Calculated VIN Check Digit is Incorrect";
	}
	// END check if the 9th character in the string (the check digit) equals the calculated value

$form_id = 1; // update to your form ID
$field_id = 7; // update to your field ID
$field_value = $vin; // VIN entry title

//$form = RGFormsModel::get_form_meta($form_id);
$field = RGFormsModel::get_field($form, $field_id);

if(RGFormsModel::is_duplicate($form_id, $field, $field_value)) {

		$validation_result["is_valid"] = false;
		$form["fields"][0]["failed_validation"] = true;
		$form["fields"][0]["validation_message"] = "This is a duplicate VIN";

}

	// check VIN length
	if (strlen($vin) != 17) {
		$validation_result["is_valid"] = false;
		$form["fields"][0]["failed_validation"] = true;
		$form["fields"][0]["validation_message"] = "Your VIN must be 17 digits in length";
	}
	// END check VIN length

	// check if the 1st and 2nd characters are WB
	if ($char[0] == W && $char[1] == B) {
	} elseif ($char[0] == w && $char[1] == b) {
		$validation_result["is_valid"] = false;
		$form["fields"][0]["failed_validation"] = true;
		$form["fields"][0]["validation_message"] = "Your VIN must begin with the letters WB and be all uppercase";
	} else {
		$validation_result["is_valid"] = false;
		$form["fields"][0]["failed_validation"] = true;
		$form["fields"][0]["validation_message"] = "Your VIN must begin with the letters WB";
	}
	// END check if the 1st and 2nd characters are WB

// update the form in the validation result with the form object you modified
$validation_result["form"] = $form;

return $validation_result;

}