gistfile1.php

<?php
/*
 * Postcode validation function

 * Based on code by John Gardner: http://www.braemoor.co.uk/software/postcodes.shtml
 * Converted to use PCRE rather than the now deprecated POSIX regular expressions by
 * David Evans.
 *
 * Optional second argument is set to the properly capitalised and spaced version of
 * the postcode.
 *
 */
function checkPostcode($toCheck, &$postcode = NULL)
{
	// Permitted letters depend upon their position in the postcode.
	$alpha1 = "[abcdefghijklmnoprstuwyz]";                          // Character 1
	$alpha2 = "[abcdefghklmnopqrstuvwxy]";                          // Character 2
	$alpha3 = "[abcdefghjkpmnrstuvwxy]";                            // Character 3
	$alpha4 = "[abehmnprvwxy]";                                     // Character 4
	$alpha5 = "[abdefghjlnpqrstuwxyz]";                             // Character 5

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

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

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

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

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

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

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

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

	// 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/', 'c/o ', $postcode);

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

	return $valid;
}