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/**
 * Custom validation callback to validate UK postcodes.
 *
 * It also tries to format provided postcode in correct format.
 *
 * Note: It's only usable for "postcode" fields.
 */
public function check_postcode_uk($original_postcode)
{
	// Set callback's custom error message (CI specific)
	// $this->set_message('check_postcode_uk', 'Invalid UK postcode format.');

	// Permitted letters depend upon their position in the postcode.
	// Character 1
	$alpha1 = "[abcdefghijklmnoprstuwyz]";
	// Character 2
	$alpha2 = "[abcdefghklmnopqrstuvwxy]";
	// Character 3
	$alpha3 = "[abcdefghjkpmnrstuvwxy]";
	// Character 4
	$alpha4 = "[abehmnprvwxy]";
	// Character 5
	$alpha5 = "[abdefghjlnpqrstuwxyz]";

	// Expression for postcodes: AN NAA, ANN NAA, AAN NAA, and AANN NAA with a space
	$pcexp[0] = '/^('.$alpha1.'{1}'.$alpha2.'{0,1}[0-9]{1,2})([[:space:]]{0,})([0-9]{1}'.$alpha5.'{2})$/';

	// Expression for postcodes: ANA NAA
	$pcexp[1] = '/^('.$alpha1.'{1}[0-9]{1}'.$alpha3.'{1})([[:space:]]{0,})([0-9]{1}'.$alpha5.'{2})$/';

	// Expression for postcodes: AANA NAA
	$pcexp[2] = '/^('.$alpha1.'{1}'.$alpha2.'{1}[0-9]{1}'.$alpha4.')([[:space:]]{0,})([0-9]{1}'.$alpha5.'{2})$/';

	// Exception for the special postcode GIR 0AA
	$pcexp[3] = '/^(gir)([[:space:]]{0,})(0aa)$/';

	// Standard BFPO numbers
	$pcexp[4] = '/^(bfpo)([[:space:]]{0,})([0-9]{1,4})$/';

	// c/o BFPO numbers
	$pcexp[5] = '/^(bfpo)([[:space:]]{0,})(c\/o([[:space:]]{0,})[0-9]{1,3})$/';

	// Overseas Territories
	$pcexp[6] = '/^([a-z]{4})([[:space:]]{0,})(1zz)$/';

	// Anquilla
	$pcexp[7] = '/^ai-2640$/';

	// Load up the string to check, converting into lowercase
	$postcode = strtolower($original_postcode);

	// Assume we are not going to find a valid postcode
	$valid = FALSE;

	// Check the string against the six types of postcodes
	foreach ($pcexp as $regexp)
	{
		if (preg_match($regexp, $postcode, $matches))
		{
			// Load new postcode back into the form element
			$postcode = strtoupper ($matches[1] . ' ' . $matches [3]);

			// Take account of the special BFPO c/o format
			$postcode = preg_replace ('/C\/O([[:space:]]{0,})/', 'c/o ', $postcode);

			// Take acount of special Anquilla postcode format (a pain, but that's the way it is)
			preg_match($pcexp[7], strtolower($original_postcode), $matches) AND $postcode = 'AI-2640';

			// Remember that we have found that the code is valid and break from loop
			$valid = TRUE;
			break;
		}
	}

	// Return with the reformatted valid postcode in uppercase if the postcode was
	return $valid ? $postcode : FALSE;
}