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/**
 * CALLBACK - determine if the provided postcode is valid.
 *
 * @param string $postcode
 * @return bool TRUE if valid, FALSE otherwise
 * @author George Edwards
 */
function is_valid_uk_postcode($postcode) {
    $pattern = "/^([A-PR-UWYZ0-9][A-HK-Y0-9][AEHMNPRTVXY0-9]?[ABEHMNPRVWXY0-9]? {1,2}[0-9][ABD-HJLN-UW-Z]{2}|GIR 0AA)$/"
 
    if (preg_match($pattern, $postcode)) {
        return TRUE;
    }
 
    $this->validation->set_message('is_valid_uk_postcode', 'That is not a valid %s.');
    return FALSE;
}