Date:: 12 May, 2007
Comments: 5 comments
: Posted by Stewart

Javascript: check if a UK postcode is valid

Here's two useful postcode functions; one that will help for checking if a string is in the valid format for a UK postcode; and another that formats a string into a nice postcode format. They both use regular expressions to make them fast! You could use them for client-side form validation.

UK postcode format

UK postcodes have a particular format of numbers and letters. You can check out the format on the government website which documents this in detail:-

http://www.cabinetoffice.gov.uk/govtalk/schemasstandards/e-gif/datastandards/address/postcode.aspx

Checking a postcode is valid

This function tests to see if the value is in the correct format for a UK postcode.

/* tests to see if string is in correct UK style postcode: AL1 1AB, BM1 5YZ etc. */
function isValidPostcode(p) {
	var postcodeRegEx = /[A-Z]{1,2}[0-9]{1,2} ?[0-9][A-Z]{2}/i;
	return postcodeRegEx.test(p);
}

Format a valid postcode

You can also use this function to format a postcode into a nice to read format:-

/*	formats a VALID postcode nicely: AB120XY -> AB1 0XY */
function formatPostcode(p) {
	if (isValidPostcode(p)) {
 var postcodeRegEx = /(^[A-Z]{1,2}[0-9]{1,2})([0-9][A-Z]{2}$)/i;
 return p.replace(postcodeRegEx,"$1 $2");
	} else {
 return p;
	}
}

Download the check if a UK postcode is valid example.