Postcode Validation

Wednesday, 23 May 2007 at 14:12 - Blog

Wikipedia cracks me up sometimes.  I'm sat here at my desk, trying to knock together some C# code to quickly validate an entered string as being a UK postcode.  Naturally I turned to this Wikipedia article for assistance, where I find this quip:

"Automatic validation of postcodes on the basis of pattern feasibility is therefore almost impossible to design, and the system contains no self-validating feature such as a check digit. Completely accurate validation is only possible by attempting to deliver mail to the address, and verifying with the recipient."

I think that's what is known as a long-running transaction!!

Update - the C# code that I wrote to handle this issue is available here.

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#1 Jez on 26 May 2007 at 05:00

Do the Post Office have a web API for postcode validation? I reckon anyone who's been coding long enough has had at least one stab at it, and for correct verification I guess the postcode is just stripped of whitespace, hashed and compared against a (huge) list of known postcodes. Easy, yeah? US zip codes are possibly a simpler scheme; any 5-digit number will do. Very easy to validate!

 

#2 Ian on 27 May 2007 at 10:39

The Royal Mail make a vast amount of money from licensing their copyrighted Postcode Address File, as do third parties such as QAS who licence this data to develop software solutions.  So no, there's no handy Web API.

 

 

All I really want is some code to *quickly* parse a string as (probably) being a valid postcode, and breaking it down into the "outer" and "inner" postcode.  I think I can achieve this using some regular expressions rather than making a costly call to a database or web service (costly in terms of performance, and licensing fees!)

 

#3 John Topley on 28 May 2007 at 05:00

I think it depends what you mean by valid postcode. You could have a postcode that's theoretically valid but that doesn't actually correspond to a real address. I wrote some VB6 code years ago at work to check the former and we had the PAF stored in SQL Server for checking the latter.

#4 Ian on 28 May 2007 at 06:08

John, you're right, I should have been more specific in my original post.  I'm knocking together some code to check that a string conforms to the definition of a postcode given in BS7666, so theoretically valid as you say.

#5 Nathan on 25 Jan 2008 at 13:02

This regular expression matches all valid postcodes, and filters out most obvious syntax errors:

/^([A-Z][A-Z]?[0-9][0-9A-Z]? *[0-9][A-Z][A-Z])$/

However, filtering out invalid postcodes isn't possible without having a list of all valid postcodes. "BH1 1QP" is invalid even though it's syntactically correct.

#6 Ryan on 13 Jun 2008 at 12:07

The wiki comment made me laugh!

Automatic validation of postcodes on the basis of pattern feasibility is therefore almost impossible to design, and the system contains no self-validating feature such as a check digit. Completely accurate validation is only possible by attempting to deliver mail to the address, and verifying with the recipient.

I am writting a .NET app that uses the QAS webservice, am not sure sending the mail to the address for validation would be sufficient in my system, not even sure how Id code that hahaha!

#7 Robert on 13 Aug 2008 at 13:00

See : www.braemoor.co.uk/.../postcodes.shtml

Free scripts for validating post codes

#8 ian on 13 Aug 2008 at 13:05

Thanks Robert, you have some nice validation scripts there.

#9 Dave R. on 01 Sep 2008 at 16:08

A more comprehensive regex is:

/^(GIR 0AA|BFPO ([0-9]{1,4})|[A-PR-UWYZ]([0-9]{1,2}|([A-HK-Y][0-9]|[A-HK-Y][0-9]([0-9]|[ABEHMNPRV-Y]))|[0-9][A-HJKS-UW]) [0-9][ABD-HJLNP-UW-Z]{2})$/

This covers BS7666 plus BFPO addresses. Unfortunately Nathan's regex will report many invalid postcodes as valid.

Valid formats (where A=alpha, 9=digit):

A9 9AA, A99 9AA, A9A 9AA, AA9 9AA, AA99 9AA, AA9A 9AA, BFPO 9, BFPO 99, BFPO 999, BFPO 9999, GIR 0AA

I recently took great satisfaction in replacing several hundred lines of crappy Javascript code with a one-liner based on this ;)

The only way to ensure that your app is 100% accurate is to subscribe to the daily PAF update from The Royal Mail. However, I believe that most of us would be happy with a quarterly feed.

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