You need a regular expression that matches a U.K. postcode.

^[A-Z]{1,2}[0-9R][0-9A-Z]?●[0-9][ABD-HJLNP-UW-Z]{2}$

Postal codes in the U.K. (or postcodes, as they’re called) are composed of five to seven alphanumeric characters separated by a space. The rules covering which characters can appear at particular positions are rather complicated and fraught with exceptions. The regular expression just shown therefore sticks to the basic rules.

If you need a regex that ticks all the boxes for the postcode rules at the expense of readability, here you go:

^(?:(?:[A-PR-UWYZ][0-9]{1,2}|[A-PR-UWYZ][A-HK-Y][0-9]{1,2}↵
|[A-PR-UWYZ][0-9][A-HJKSTUW]|[A-PR-UWYZ][A-HK-Y][0-9]↵
[ABEHMNPRV-Y])●[0-9][ABD-HJLNP-UW-Z]{2}|GIR 0AA)$

British Standard BS7666, available at http://interim.cabinetoffice.gov.uk/govtalk/schemasstandards/e-gif/datastandards/address/postcode.aspx, describes the U.K. postcode rules.

The Royal Mail’s website at http://www.royalmail.com/postcode-finder lets you use an address to look up an individual postcode.

Recipes 4.14 and 4.15 show how to validate U.S. ZIP codes and Canadian postal codes.

Recipe 4.17 explains how to identify addresses that contain a P.O. box.

Techniques used in the regular expressions in this recipe are discussed in Chapter 2. Recipe 2.3 explains character classes. Recipe 2.5 explains anchors. Recipe 2.8 explains alternation. Recipe 2.9 explains grouping. Recipe 2.12 explains repetition.