We are often required to validate input fields. One field that is often neglected or relies heavily on users inputting data in the correct format is the UK postcode field. The format of the postcode is listed here http://www.govtalk.gov.uk/gdsc/html/frames/PostCode.htm.

We have a Regular Expression / Javascript solution that validates UK postcode formats. Briefly all valid formats are:

The regex we use is:
/^([A-PR-UWYZ0-9][A-HK-Y0-9][AEHMNPRTVXY0-9]?[ABEHMNPRVWXY0-9]? {1,2}[0-9][ABD-HJLN-UW-Z]{2}|GIR 0AA)$/i

Or you could try this:
^((([A-PR-UWYZ][0-9])|([A-PR-UWYZ][0-9][0-9])|([A-PR-UWYZ][A-HK-Y][0-9])|([A-PR-UWYZ][A-HK-Y][0-9][0-9])
|([A-PR-UWYZ][0-9][A-HJKSTUW])|([A-PR-UWYZ][A-HK-Y][0-9][ABEHMNPRVWXY]))\s?([0-9][ABD-HJLNP-UW-Z]{2})|
(GIR)\s?(0AA))$

Simple and effective.

UDPATE 2009 This will only validate UPPERCASE input. You will need to upper your input before validating it.