Python Regular Expression for Outward UK Post Code

Question

I've looked and tried the solutions of previous questions on this topic (here and here), but I can't get it to work.

I'm looking for a regex for the outer part of a UK postcode. In "PO1 1AF", PO1 is the outward postcode or postcode district, with 1AF being the inward postcode. I have a long list of urls, some of which have an outer post code at the end of them.

E.g, I want "ab15" and "dd9" from these two strings:

string1= "www.xyz.com/abcdab15/"
string2 = "www.xyz.com/adbdd9"

The permutations for the outer post code are:

A9
A9A
A99
AA9
AA9A
AA99

I tried this solution from a previous answer, which is meant to match either the inner, outer or both but it doesn't return anything (the answer was for capitalised letters):

exp = '^((([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]))) || ^((GIR)[ ]?(0AA))$|^(([A-PR-UWYZ][0-9])[ ]?([0-9][ABD-HJLNPQ-UW-Z]{0,2}))$|^(([A-PR-UWYZ][0-9][0-9])[ ]?([0-9][ABD-HJLNPQ-UW-Z]{0,2}))$|^(([A-PR-UWYZ][A-HK-Y0-9][0-9])[ ]?([0-9][ABD-HJLNPQ-UW-Z]{0,2}))$|^(([A-PR-UWYZ][A-HK-Y0-9][0-9][0-9])[ ]?([0-9][ABD-HJLNPQ-UW-Z]{0,2}))$|^(([A-PR-UWYZ][0-9][A-HJKS-UW0-9])[ ]?([0-9][ABD-HJLNPQ-UW-Z]{0,2}))$|^(([A-PR-UWYZ][A-HK-Y0-9][0-9][ABEHMNPRVWXY0-9])[ ]?([0-9][ABD-HJLNPQ-UW-Z]{0,2}))$'

import re

url1= "www.xyz.com/abcdAB15/"
url2 = "www.xyz.com/adbDD9"

postalCode = re.findall(exp,url1)
print postalCode[0]

Here is the expression free of all $ and ^ anchors as suggested below:

exp = '((([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]))) || ((GIR)[ ]?(0AA))|(([A-PR-UWYZ][0-9])[ ]?([0-9][ABD-HJLNPQ-UW-Z]{0,2}))|(([A-PR-UWYZ][0-9][0-9])[ ]?([0-9][ABD-HJLNPQ-UW-Z]{0,2}))|(([A-PR-UWYZ][A-HK-Y0-9][0-9])[ ]?([0-9][ABD-HJLNPQ-UW-Z]{0,2}))|(([A-PR-UWYZ][A-HK-Y0-9][0-9][0-9])[ ]?([0-9][ABD-HJLNPQ-UW-Z]{0,2}))|(([A-PR-UWYZ][0-9][A-HJKS-UW0-9])[ ]?([0-9][ABD-HJLNPQ-UW-Z]{0,2}))|(([A-PR-UWYZ][A-HK-Y0-9][0-9][ABEHMNPRVWXY0-9])[ ]?([0-9][ABD-HJLNPQ-UW-Z]{0,2}))'

Answer 1

Given the possibilities you list for an outer postcode, it can be described as:

• one or two letters
• followed by a digit
• optionally followed by a letter or digit

Which, in regex terms, is this:

[a-z]{1,2}[0-9][a-z0-9]?

... but you only want to find that pattern at the end of the URL (possibly followed by a slash), so we'll add a lookahead:

[a-z]{1,2}[0-9][a-z0-9]?(?=/?$)

The full-postcode regex in your question incorporates a number of different exclusions. For example, it looks like V, Q and X aren't allowed in some places, and there are apparently other limitations - I won't bother to try to replicate those (reading other people's regexes is never fun) ... but using what we have:

>>> import re
>>> postcode = re.compile("[a-z]{1,2}[0-9][a-z0-9]?(?=/?$)")
>>> string1= "www.xyz.com/abcdab15/"
>>> string2 = "www.xyz.com/adbdd9"
>>> re.findall(postcode, string1)
['ab15']
>>> re.findall(postcode, string2)
['dd9']

Answer 2

The problem is the ^$ anchors, which anchor the start and end of the string respectively, meaning that the regex will only match an entire string. Remove them from each alternation (exp split on |) and it will work.