Validate & parse UK postcodes

Utility methods for UK Postcodes, including validating the shape of a postcode, extracting postcode elements (like incodes, outcodes, areas and more).

Tested against ~1.7 million postcodes on ONSPD.

Features

• Check whether a postcode conforms to the correct format
• Single purpose static methods
• Tested against a list of ~1.7 million postcodes listed on ONS Postcode Directory
• Extract useful elements of a postcode like incode, outcode, sector
• Tree-shakeable

Links

• GitHub Repository
• API Documentation
• Try postcode.js on RunKit
• Postcode element definitions
• Notes
• NPM Package

Guides

• UK Postcode Format. An overview of the UK postcode format and its constituent parts
• Postcode Validation. An overview of the many approaches to postcode validation and the tradeoffs

Getting Started

Installation

npm install postcode

Validate

import { isValid } from "postcode";

isValid("AA1 1AB"); // => true

Parse

Pass a string to parse. This will return a valid or invalid postcode instance which can be easily destructured.

Valid Postcode

ValidPostcode type definition

import { parse } from "postcode";

const {
  postcode,    // => "SW1A 2AA"
  outcode,     // => "SW1A"
  incode,      // => "2AA"
  area,        // => "SW"
  district,    // => "SW1"
  unit,        // => "AA"
  sector,      // => "SW1A 2"
  subDistrict, // => "SW1A"
  valid,       // => true
} = parse("Sw1A     2aa");

Invalid Postcode

InvalidPostcode type definition

const {
  postcode,    // => null
  outcode,     // => null
  incode,      // => null
  area,        // => null
  district,    // => null
  unit,        // => null
  sector,      // => null
  subDistrict, // => null
  valid,       // => false
} = parse("    Oh no, ):   ");

Type Guard

The TypeScript compiler can infer if you have a valid postcode type from parse by checking the valid attribute

import { parse } from "postcode";

const postcode = parse("SW1A 2AA");

if (postcode.valid) {
  // `postcode` adheres to the `ValidPostcode` interface
  processString(postcode.outcode.toLowerCase()); // TypeScript compiler knows `outcode` to be a string
  processString(postcode.subDistrict.toLowerCase()); // And it will throw errors on common gotchas (e.g. subdistrict can be `null` on a valid postcode)
} else {
  // `postcode` adheres to the `InvalidPostcode` interface
  processInvalidPostcode(postcode);
}

Valid Postcode Object

Postcode	.outcode	.incode	.area	.district	.subDistrict	.sector	.unit
AA9A 9AA	AA9A	9AA	AA	AA9	AA9A	AA9A 9	AA
A9A 9AA	A9A	9AA	A	A9	A9A	A9A 9	AA
A9 9AA	A9	9AA	A	A9	null	A9 9	AA
A99 9AA	A99	9AA	A	A99	null	A99 9	AA
AA9 9AA	AA9	9AA	AA	AA9	null	AA9 9	AA
AA99 9AA	AA99	9AA	AA	AA99	null	AA99 9	AA

Exported Methods

If you're just after a single value, you can import a single method.

Validation

isValid("Sw1A 2aa"); // => true

Formatting

import {
  toNormalised,
  toOutcode,
  toIncode,
  toArea,
  toDistrict,
  toSubDistrict,
  toSector,
  toUnit,
} from "postcode";

toNormalised("Sw1A 2aa");  // => "SW1A 2AA"
toOutcode("Sw1A 2aa");     // => "SW1A"
toIncode("Sw1A 2aa");      // => "2AA"
toArea("Sw1A 2aa");        // => "SW"
toDistrict("Sw1A 2aa");    // => "SW1"
toSubDistrict("Sw1A 2aa"); // => "SW1A"
toSector("Sw1A 2aa");      // => "SW1A 2"
toUnit("Sw1A 2aa");        // => "AA"

Fix

fix Attempts to correct and clean up a postcode without validating by replacing commonly misplaced characters (e.g. mixing up 0 and "O", 1 and "I"). This method will also uppercase and fix spacing. The original input is returned if it cannot be reliably fixed.

fix("SWIA 2AA") => "SW1A 2AA" // Corrects I to 1
fix("SW1A 21A") => "SW1A 2IA" // Corrects 1 to I
fix("SW1A OAA") => "SW1A 0AA" // Corrects O to 0
fix("SW1A 20A") => "SW1A 2OA" // Corrects 0 to O

// Other effects
fix(" SW1A  2AO") => "SW1A 2AO" // Properly spaces
fix("sw1a 2aa") => "SW1A 2AA" // Uppercase

Aims to be used in conjunction with parse to make postcode entry more forgiving:

const { inward } = parse(fix("SW1A 2A0")); // inward = "2AO"

If the input is not deemed fixable, the original string will be returned

fix("12a") => "12a"

Extract & Replace

match. Retrieve valid postcodes in a body of text

const matches = match("The PM and her no.2 live at SW1A2aa and SW1A 2AB"); // => ["SW1A2aa", "SW1A 2AB"]

// Perform transformations like normalisation using `.map` and `toNormalised`
matches.map(toNormalised); // => ["SW1A 2AA", "SW1A 2AB"]
matches.map(toOutcode); // => ["SW1A", "SW1A"]

// No matches yields empty array
match("Some London outward codes are SW1A, NW1 and E1"); // => []

replace. Replace postcodes in a body of text, returning the updated corpus and any matching postcodes

const { match, result } = replace("The PM and her no.2 live at SW1A2AA and SW1A 2AB");
// => match: ["SW1A2AA", "SW1A 2AB"]
// => result: "The PM and her no.2 live at  and "

// Add custom replacement
replace("The PM lives at SW1A 2AA", "Downing Street");
// => { match: ["SW1A 2AA"], result: "The PM lives at Downing Street" };

// No match
replace("Some London outward codes are SW1A, NW1 and E1");
// => { match: [], result: "Some London outward codes are SW1A, NW1 and E1" }

Version 5.0.0

5.0.0 brings changes which allows for better treeshaking and interopability with ES Modules. It also deprecates legacy class based APIs in favour of single purpose methods.

Breaking Changes

• postcode no longer exports a class. Legacy new Postcode() functionality has been removed. Methods attached to Postcode are all available as named exports.
• postcode no longer uses default exports. All exports are named. E.g.

// In <= 4.0.0
import Postcode from "postcode";
Postcode.parse("SW1A 2AA");

// In >= 5.0.0
import { parse } from "postcode";
parse("SW1A 2AA");

In many cases, migration can be achieved by changing import Postcode from "postcode" to import * as Postcode from "postcode", however this gives up treeshaking advantages.

New Features

• postcode now exports a ES Module build
• Exports regular expressions
• match accepts a string and returns all valid postcodes
• replace accepts a string and replaces valid postcodes with an optional second argument. Default replacement text is empty string ""

Definitions

See the postcode format guide for a glossary of postcode component terms.

Notes

Postcodes cannot be validated just with a regular expression (however complex). True postcode validation requires having a full list of postcodes to check against. Relying on a regex will produce false postives/negatives.

See the postcode validation guide for an overview of the approaches and tradeoffs associated with postcode validation.

Testing

npm test

License

MIT

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