PHP: Split a postcode into two parts?

Question

I need to split a UK postcode into two. I have some code that gets the first half but it doesn't cover everything (such as gir0aa). Does anyone have anything better that validates all UK postcodes then breaks it into the first and second half? Thanks.

function firstHalf($postcode) {
if(preg_match('/^(([A-PR-UW-Z]{1}[A-IK-Y]?)([0-9]?[A-HJKS-UW]?[ABEHMNPRVWXY]?|[0-9]?[0-9]?))\s?([0-9]{1}[ABD-HJLNP-UW-Z]{2})$/i',$postcode))
    return preg_replace('/^([A-Z]([A-Z]?\d(\d|[A-Z])?|\d[A-Z]?))\s*?(\d[A-Z][A-Z])$/i', '$1', $postcode);
}

will split ig62ts into ig6 or cm201ln into cm20.

Answer 1

The incode is always a single digit followed by two alpha characters, so the easiest way to split is to chop off the last three characters, allowing it to be validated easily.

Trim any spaces: they're used purely for ease of human readability.

The first part that remains is then the outcode. This can be a single alpha character followed by 1 or 2 digits; two alpha characters followed by 1 or 2 digits; or one or two characters followed by a single digit, followed by an additional alpha character.

There are a couple of notable exceptions: SAN TA1 is a recognised postcode, as is GIR 0AA; but these are the only two that don't follow the standard pattern.

To test if a postcode is valid, a regexp isn't really adequate... you need to do a lookup to retrieve that information.