UK postcode parsing and validation for Ruby
Ruby Makefile
Latest commit c22dee5 Jul 11, 2017 @threedaymonk Correct typo in README


UK postcode parsing and validation for Ruby for writing friendly software.


  • Handles errors with I/1 and O/0.
  • Does not require postcodes to contain spaces.
  • Normalises postcodes (e.g. wiaiaa to W1A 1AA).
  • Parses full postcodes or outcodes (W1A)
  • Allows extraction of fields within postcode.
  • Validated against 2.5 million postcodes in England, Wales, Scotland, Northern Ireland, the Channel Islands, and the Isle of Man.
  • Finds the country corresponding to a postcode, where possible.

Note: There's a distinction between validity and existence. This library validates the format of a postcode, but not whether it actually currently refers to a location.

By analogy, is a valid email address in terms of format, but you can't successfully deliver an email to it.


require "uk_postcode"

Parse and extract sections of a full postcode:

pc = UKPostcode.parse("W1A 2AB")
pc.valid?      # => true
pc.full?       # => true
pc.full_valid? # => true
pc.outcode     # => "W1A"
pc.incode      # => "2AB"
pc.area        # => "W"
pc.district    # => "1A"
pc.sector      # => "2"
pc.unit        # => "AB"

Or of a partial postcode:

pc = UKPostcode.parse("W1A")
pc.valid?      # => true
pc.full?       # => false
pc.full_valid? # => false
pc.outcode     # => "W1A"
pc.incode      # => nil
pc.area        # => "W"
pc.district    # => "1A"
pc.sector      # => nil
pc.unit        # => nil

Postcodes are converted to a normal or canonical form:

pc = UKPostcode.parse("w1a1aa")
pc.valid?      # => true
pc.area        # => "W"
pc.district    # => "1A"
pc.sector      # => "1"
pc.unit        # => "AA
pc.to_s        # => "W1A 1AA"

And mistakes with I/1 and O/0 are corrected:

pc = UKPostcode.parse("WIA OAA")
pc.valid?      # => true
pc.area        # => "W"
pc.district    # => "1A"
pc.sector      # => "0"
pc.unit        # => "AA
pc.to_s        # => "W1A 0AA"

Find the country of a full or partial postcode (if possible: some outcodes span countries):

UKPostcode.parse("W1A 1AA").country # => :england
UKPostcode.parse("BT4").country # => :northern_ireland
UKPostcode.parse("CA6").country # => :unknown
UKPostcode.parse("CA6 5HS").country # => :scotland
UKPostcode.parse("CA6 5HT").country # => :england

The country returned for a postcode is derived from the ONS Postcode Directory and might not always be correct in a border region:

Users should note that postcodes that straddle two geographic areas will be assigned to the area where the mean grid reference of all the addresses within the postcode falls.

Invalid postcodes:

pc = UKPostcode.parse("Not Valid")
pc.valid?      # => false
pc.full?       # => false
pc.full_valid? # => false
pc.area        # => nil
pc.to_s        # => "Not valid"     # => :unknown

Tips for Rails

You can normalise postcodes on assignment by overriding a setter method (this assumes that you have a postcode field on the model):

def postcode=(str)
  super UKPostcode.parse(str).to_s

Invalid postcodes are parsed to instances of InvalidPostcode, whose #to_s method gives the original input, so an invalid postcode will be presented back to the user as originally entered.

To validate, use something like this:

class PostcodeValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    ukpc = UKPostcode.parse(value)
    unless ukpc.full_valid?
      record.errors[attribute] << "not recognised as a UK postcode"

class Address
  validates :postcode, presence: true, postcode: true

For users of version 1.x

The interface has changed significantly, so code that worked with version 1.x will not work with version 2.x without changes.


  • Use UKPostcode.parse(str) where you previously used
  • parse will return either a GeographicPostcode, a GiroPostcode, or an InvalidPostcode.
  • You may prefer to use GeographicPostcode.parse directly if you wish to exclude GIR 0AA and invalid postcodes.

As a gem

In your Gemfile:

gem "uk_postcode", "~> 2.1.0"


To run the test suite:

$ make

The full list of UK postcodes is not included in the repository due to its size, but will be fetched automatically from mySociety.

If you are running an automatic build process, please find a way to cache these files and run the tests via Rake instead:

$ rake

Occasionally asked questions

Does it support Father Christmas's postcode?

No. The old postcode was SAN TA1; the current one is XM4 5HQ. (XMAS HQ, geddit?) For most people, these probably aren't useful, as they don't correspond to actual locations, and are only used by Royal Mail internally.

See "Adding additional formats" if you'd like to support this.

Does it support BFPO?

No. They're not really postcodes, though they serve a similar purpose. Some of them are abroad; some of them are on boats that move around; some of them are ephemeral and exist only for particular operations. This library has been designed with the assumption that most people won't want to handle BFPO codes, and that those that do can do so explicitly.

See "Adding additional formats" if you'd like to support them.

The new BF1 format postcodes can be parsed, although their location is always unknown.

Adding additional formats

Parsing is implemented via the ParserChain class, which attempts to parse the supplied text via each parser in turn. The UKPostcode.parse method is effectively a thin wrapper that does this:

chain =, GeographicPostcode, InvalidPostcode)

Each class passed to must implement a class method parse(str) and return either a postcode object (see AbstractPostcode) or nil. The first non-nil object is returned.

InvalidPostcode is at the end of the chain to ensure that a postcode object is always returned.

To add an additional class, subclass AbstractPostcode, implement the abstract methods, and instantiate your own ParserChain.


You may use this library according to the terms of the MIT License; see COPYING.txt for details.

The regular expressions in country_lookup.rb are derived from the ONS Postcode Directory according to the terms of the Open Government Licence.

Under the terms of the Open Government Licence (OGL) […] anyone wishing to use or re-use ONS material, whether commercially or privately, may do so freely without a specific application for a licence, subject to the conditions of the OGL and the Framework. Users reproducing ONS content must include a source accreditation to ONS.

In order to avoid the restrictive commercial terms of the Northern Ireland data in the ONSPD, this is not used to generate the regular expressions. Fortunately, Northern Ireland postcodes are very simple: they all start with BT!