Reading a Cookie using RegExp

JavaScript FAQ | JavaScript Cookies FAQ  

Question: How do I retrieve a cookie with a given name using a regular expression?

Answer: To read a cookie cookieName that has been set by the setCookie function, you can use the following RegExp-based function:

function readCookie(cookieName) {
 var re = new RegExp('[; ]'+cookieName+'=([^\\s;]*)');
 var sMatch = (' '+document.cookie).match(re);
 if (cookieName && sMatch) return unescape(sMatch[1]);
 return '';
Cookie Name: 
This readCookie function is somewhat less readable than the non-RegExp version - but, as we will see, it is more flexible. Let us walk through the function code, line by line.

Code walk-through:  The readCookie function takes cookieName as a parameter. First, we call the RegExp constructor to create a regular expression for finding a cookie by name:

var re = new RegExp('[; ]'+cookieName+'=([^\\s;]*)');
// to get \s in regular expression, pass \\s to RegExp constructor!
Our regular expression re is constructed to match the desired cookie name preceded by a semicolon or a space [; ] and followed by an equal sign = and (immediately after =) a substring of non-whitespace non-semicolon characters [^\s;]*. This non-whitespace non-semicolon substring, if found, will be retained in the capturing group ([^\s;]*). The substring captured in the group will be precisely the cookie value part of document.cookie.
var sMatch = (' '+document.cookie).match(re);
In this line of code, we use the match() method to find the fragment of the string (' '+document.cookie) that matches the RegExp we just created. The value returned by match() is stored in the sMatch variable. If there is no matching cookie, sMatch will be set to null. If there is a match, sMatch will store an array of matched substrings: the first element sMatch[0] contains the whole RegExp match (e.g. MyCookie=CookieValue). Subsequent elements of the array, sMatch[1] etc., will hold the substrings matched with each capturing group. Since we have only one capturing group, sMatch[1] is the last element in the array; it is in sMatch[1] that match() puts the cookie value we want to get.
if (cookieName && sMatch) return unescape(sMatch[1]);
return '';
Here, before returning the cookie value, we make sure that (1) cookieName is not empty and (2) there was actually a match. The if condition combines the values cookieName and sMatch in a Boolean context; so the if condition is true when cookieName is a non-empty string and sMatch is not null. If so, we return the (unescaped) cookie value in sMatch[1]. Otherwise, that is, when cookieName is empty or sMatch is null (no match), the function returns an empty string.

Final remarks:  Our new function is easy to modify. For example, to support not only alphanumeric cookieName but also (exceedingly rare) cookie names with "dangerous" characters, we could add code preparing a potentially "bad" cookieName for use in the RegExp constructor:


Or would you like to require only non-empty cookie values? No problem: just replace the * (match zero or more characters) with + (match one or more characters) in the RegExp capturing group ([...]*). Or maybe you want to allow not only semicolons but also commas, as cookie separators? Then simply use the characters ,; instead of just ; in your RegExp constructor call:

var re = new RegExp('[,; ]'+cookieName+'=([^\\s,;]*)')

Other potential modifications of the readCookie function are also straightforward, thanks to the flexibility of regular expressions. (Please refer to Regular Expressions Syntax for a review of the RegExp elements such as [^...], *, +, or capturing groups in regular expressions.)



Copyright © 1999-2012,