United Kingdom (GB) postal code validation without regex

Question

I have tried several regexes and still some valid postal codes sometimes get rejected.

Searching the internet, Wikipedia and SO, I could only find regex validation solutions.

Is there a validation method which does not use regex? In any language, I guess it would be easy to port.

I supose the easiest would be to compare against a postal code database, yet that would need to be maintained and updated periodically from a reliable source.

Edit: To help future visitors and keep you from posting any more regexes, here's a regex which I have tested (as of 2013-04-24) to work for all postal codes in Code Point (see @Mikkel Løkke's answer):

//PHP PCRE (it was on Wikipedia, it isn't there anymore; I might have modified it, don't remember).
$strPostalCode=preg_replace("/[\s]/", "", $strPostalCode);
$bValid=preg_match("/^(GIR 0AA)|(((A[BL]|B[ABDHLNRSTX]?|C[ABFHMORTVW]|D[ADEGHLNTY]|E[HNX]?|F[KY]|G[LUY]?|H[ADGPRSUX]|I[GMPV]|JE|K[ATWY]|L[ADELNSU]?|M[EKL]?|N[EGNPRW]?|O[LX]|P[AEHLOR]|R[GHM]|S[AEGKLMNOPRSTY]?|T[ADFNQRSW]|UB|W[ADFNRSV]|YO|ZE)[1-9]?[0-9]|((E|N|NW|SE|SW|W)1|EC[1-4]|WC[12])[A-HJKMNPR-Y]|(SW|W)([2-9]|[1-9][0-9])|EC[1-9][0-9])[0-9][ABD-HJLNP-UW-Z]{2})$/i", $strPostalCode);

Answer 1

I'm writing this answer based on the wiki page.

When checking on the validation part, it seems that there are 6 type of formats (A = letter and 9 = digit):

AA9A 9AA                       AA9A9AA                   AA9A9AA
A9A 9AA     Removing space     A9A9AA       order it     AA999AA
A9 9AA    ------------------>  A99AA     ------------->  AA99AA
A99 9AA                        A999AA                    A9A9AA
AA9 9AA                        AA99AA                    A999AA
AA99 9AA                       AA999AA                   A99AA

As we can see, the length may vary from 5 to 7 and we have to take in account some special cases if we want to.

So the function we are coding has to do the following:

1. Remove spaces and convert to uppercase (or lower case).
2. Check if the input is an exception, if it is it should return valid
3. Check if the input's length is 4 < length < 8.
4. Check if it's a valid postcode.

The last part is tricky, but we will split it in 3 sections by length for some overview:

1. Length = 7: AA9A9AA and AA999AA
2. Length = 6: AA99AA, A9A9AA and A999AA
3. Length = 5: A99AA

For this we will be using a switch(). From now on it's just a matter of checking character by character if it's a letter or a number on the right place.

So let's take a look at our PHP implementation:

function check_uk_postcode($string){
    // Start config
    $valid_return_value = 'valid';
    $invalid_return_value = 'invalid';
    $exceptions = array('BS981TL', 'BX11LT', 'BX21LB', 'BX32BB', 'BX55AT', 'CF101BH', 'CF991NA', 'DE993GG', 'DH981BT', 'DH991NS', 'E161XL', 'E202AQ', 'E202BB', 'E202ST', 'E203BS', 'E203EL', 'E203ET', 'E203HB', 'E203HY', 'E981SN', 'E981ST', 'E981TT', 'EC2N2DB', 'EC4Y0HQ', 'EH991SP', 'G581SB', 'GIR0AA', 'IV212LR', 'L304GB', 'LS981FD', 'N19GU', 'N811ER', 'NG801EH', 'NG801LH', 'NG801RH', 'NG801TH', 'SE18UJ', 'SN381NW', 'SW1A0AA', 'SW1A0PW', 'SW1A1AA', 'SW1A2AA', 'SW1P3EU', 'SW1W0DT', 'TW89GS', 'W1A1AA', 'W1D4FA', 'W1N4DJ');
    // Add Overseas territories ?
    array_push($exceptions, 'AI-2640', 'ASCN1ZZ', 'STHL1ZZ', 'TDCU1ZZ', 'BBND1ZZ', 'BIQQ1ZZ', 'FIQQ1ZZ', 'GX111AA', 'PCRN1ZZ', 'SIQQ1ZZ', 'TKCA1ZZ');
    // End config


    $string = strtoupper(preg_replace('/\s/', '', $string)); // Remove the spaces and convert to uppercase.
    $exceptions = array_flip($exceptions);
    if(isset($exceptions[$string])){return $valid_return_value;} // Check for valid exception
    $length = strlen($string);
    if($length < 5 || $length > 7){return $invalid_return_value;} // Check for invalid length
    $letters = array_flip(range('A', 'Z')); // An array of letters as keys
    $numbers = array_flip(range(0, 9)); // An array of numbers as keys

    switch($length){
        case 7:
            if(!isset($letters[$string[0]], $letters[$string[1]], $numbers[$string[2]], $numbers[$string[4]], $letters[$string[5]], $letters[$string[6]])){break;}
            if(isset($letters[$string[3]]) || isset($numbers[$string[3]])){
                return $valid_return_value;
            }
        break;
        case 6:
            if(!isset($letters[$string[0]], $numbers[$string[3]], $letters[$string[4]], $letters[$string[5]])){break;}
            if(isset($letters[$string[1]], $numbers[$string[2]]) || isset($numbers[$string[1]], $letters[$string[2]]) || isset($numbers[$string[1]], $numbers[$string[2]])){
                return $valid_return_value;
            }
        break;
        case 5:
            if(isset($letters[$string[0]], $numbers[$string[1]], $numbers[$string[2]], $letters[$string[3]], $letters[$string[4]])){
                return $valid_return_value;
            }
        break;
    }

    return $invalid_return_value;
}

Note that I've not added British Forces Post Office and non-geographic codes.

Usage:

echo check_uk_postcode('AE3A 6AR').'<br>'; // valid
echo check_uk_postcode('Z9 9BA').'<br>'; // valid
echo check_uk_postcode('AE3A6AR').'<br>'; // valid
echo check_uk_postcode('EE34      6FR').'<br>'; // valid
echo check_uk_postcode('A23A 7AR').'<br>'; // invalid
echo check_uk_postcode('A23A   7AR').'<br>'; // invalid
echo check_uk_postcode('WA3334E').'<br>'; // invalid
echo check_uk_postcode('A2 AAR').'<br>'; // invalid

Answer 2

As supplied by the UK government.

   (GIR 0AA)|((([A-Z-[QVX]][0-9][0-9]?)|(([A-Z-[QVX]][A-Z-[IJZ]][0-9][0-9]?)|(([A-Z-[QVX]][0-9][A-HJKSTUW])|([A-Z-[QVX]][A-Z-[IJZ]][0-9][ABEHMNPRVWXY])))) [0-9][A-Z-[CIKMOV]]{2})

I've built London only postcode based apps using the postcodes I got from HERE. But to be honest, even with London postcodes only, you need a lot more storage than necessary. Sure, the idea is trivial.

Store the postcodes, take the user input or whatever, and see if you get a match. But you are complicating the solution far more than you think. I HAD to use actual postcodes to achieve what I wanted, but for simple validation purposes, as hard as "maintaining" a regex is, storing tens of thousands or hundreds of thousands(if not more) and validating more or less in real-time is a far more difficult task.

If a mini distributed service sounds like a more efficient solution than a regex, go for it, but I'm sure it isn't. Unless you need geo-spatial querying of your own data against UK postcodes or things like that, I doubt DB storage is a feasible solution. Just my 2 cents.

Update

According to this index, there are 1,758,417 postcodes in the UK. I can tell you I am using a few Mongo clusters (Amazon EC2 High Memory Instances) to provide reliable London only services(indexing only London postcodes), and it's quite a pricy thing, even with basic storage.

Admittedly, the app is performing medium complexity geo-spatial queries, but the storage requirements alone are very expensive and demanding.

Bottom line, just stick to regex and be done with it in two minutes.