Tony Finch – blog

A little contribution for anyone else who searches the web for this in the future.

The UK postcode consists of two parts. The first part is the Outward Postcode, or Outcode. This is separated by a single space from the second part which is the Inward Postcode, or Incode. The Outcode directs mail to the correct local area for delivery. The Incode is used to sort the mail at the local area delivery office.

The Outcode has 6 possible formats (as follows) and the Incode is consistently numeric, alpha, alpha format.

• AN NAA
• ANN NAA
• ANA NAA
• AAN NAA
• AANN NAA
• AANA NAA

There are some restrictions on the letters:

1. The letters [QVX] are not used in the first position.
2. The letters [IJZ] are not used in the second position.
3. The only letters to appear in the third position are [ABCDEFGHJKSTUW].
4. The only letters to appear in the fourth position are [ABEHMNPRVWXY].
5. The letters [CIKMOV] are not used in the second part.

This translates into a perl extended regex as follows (with slightly relaxed whitespace):

qr{\b
    ([A-PR-UWYZ]\d[\dA-HJKSTUW]? # rules 1,3
    |[A-PR-UWYZ][A-HK-Y]\d[\dABEHMNPRVWXY]? # rules 1,2,4
    )[\t ]{1,2}
    (\d[ABD-HJLNP-UW-Z]{2}) # rule 5
\b}x

Update: more here.