ukpcre

Flexible regular expression to detect full or partial UK postcodes.

Structure.

The regular expression is based both on official versions and variatons thereof. It looks like this (adding spacing and indentation to make it easier to read):

^(?P<postcode>
	(?P<girobank>gir\s*0aa) |                            # Girobank.
	(
		(?P<first>
			((?![qvx])[a-z][a-hk-y]?[0-9][0-9]?) |       # First part: A1, AB1, A12, AB12.
			((?![qvx])[a-z][0-9][a-hjkpstuw]) |          # First part: A1B.
			((?![qvx])[a-z][a-hk-y][0-9][abehmnprvwxy])  # First part: AB1C.
		)\s*(?P<second>[0-9](?![cikmov])[a-z]{2})?       # Second part.
	)
)$

It can be divided into three distinct blocks. In all cases, both uppercase and lowercase letters are accepted, and spaces are optional where relevant.

Girobank postcode.

The first group matches the Girobank postcode (GIR 0AA).

First section.

The first part of a postcode is covered by two distinct cases:

1. It's followed by the second part, with or without one (or more) whitespace(s).
2. It's the only part of the postcode provided.

Three possibilities are covered via sub-expressions:

1. Postcodes in the form A1, AB1, A12 or AB12: (?![qvx])[a-z][a-hk-y]?[0-9][0-9]?.
2. Postcodes in the form A1B: (?![qvx])[a-z][0-9][a-hjkpstuw].
3. Postcodes in the form AB1C: (?![qvx])[a-z][a-hk-y][0-9][abehmnprvwxy].

In all cases, the corresponding alphabetic character classes include the appropriate exclusions, as defined in the relevant Wikipedia article.

Second section.

The final section of the full postcode sub-expression, ([0-9](?![cikmov])[a-z]{2}), matches the second half of a postcode, with or without leading whitespaces (\s*). It starts by a number, followed by two letters, excluding the characters [cikmov].

Testing.

A test suite is provided, with cases to match all of the cases mentioned, including character exclusions. It can be run using py.test.

For every failure a line is generated in a file called failures.list, including the affected postcode. This way, it's fairly easy to see a list of the offending postcodes.

Testing against OS database.

A script and a test generator is provided for testing against the Ordnance Survey Code-Point Open postcode database. This database can't be provided with the code, though, but can be obtained through the official channels.

Once the database has been obtained, the ZIP file can be decompressed under the tests/data/ directory, and the script tests/prepare_os_data.py will generate the postcode database for you. If the test suite is run using the --osdb command line switch it will use it. Beware, though: it generates one test for every postcode, and it will take a while.

Invoke tasks.

A tasks.py file is included, which uses the facilities provided by Invoke in order to provide a cross-platform Makefile-like system. The following tasks are included:

• test: Run the test suite. Accepts two arguments:
  • --osdb: Run tests against the Ordnance Survey dataset.
  • --report: Generate a JUnit-like XML report.
• genosdb: Generate the postcode database file necessary to run the test suite against the Ordnance Survey dataset.
• lint: Run the flake8 linter.
• clean: Clean up temporary files.

Notes.

You can also analyse this regex.