Encoding and Decoding URL strings

by Manohar Kamath
August 31, 1999

When you pass something as an URL's querystring, the string is encoded and all special characters like space, %, etc. are converted into their %xx values. Here, xx denotes a number and the %xx represents any character that can not be passed within the URL in their original form. The characters include spaces, all punctuation symbols, accented characters (like ? ? etc.) and non-ASCII characters(Source: MSDN library).

The URL has to be in one piece and symbols spaces can break it. Also, symbols like &, = and + are used within an URL to separate the querystring's key. So, if you want to pass a key-value pair something like Message=Hello World, the URL (when not encoded) may look like:

Listing 1
mypage.asp?mesage=Hello World!

and the space between "Hello" and "World" breaks the URL. So when you do something like

Listing 2
<%=Request.QueryString("message") %>

within mypage.asp, you will get only "Hello"

The escape() function in JavaScript parses a string and converts and special characters into their %xx equivalents. This function is very useful when you are trying to "build" a URL from within your ASP page. So if you want to build a correct version of the URL shown in listing 1, you would do something like in listing 3 (which is in JavaScript):

Listing 3
lsURL = "mypage.asp?message=" + escape("Hello World!")

Alternatively, you can use the URLEncode function in the Server object to get the same results. Listing 4 is in VBScript.

Listing 4
lsURL = "mypage.asp?message=" & Server.URLEncode("Hello World!")

The result is mypage.asp?message=Hello%20World%21

Decoding the querystring is easy - just call unescape() function in JavaScript. VBScript, on the other hand, does not have any in-built function to decode URLStrings.

Note: You will need to call unescape() within the same page as you called escape() to decode the strings. Otherwise, a simple Request.QueryString("message") will automatically decode the URL for you.

I wrote this tiny function so you can include it on any ASP page with any scripting language and use it. Just include it in any ASP page and call the function URLEncode() with a string argument.

Listing 5
<script language=JavaScript RUNAT=SERVER>
// This function decodes the any string
// that's been encoded using URL encoding technique
function URLDecode(psEncodeString)
{
  return unescape(psEncodeString);
}
</script>

An example of using this is in Listing 6

Listing 6
<%=URLDecode("Hello%20World%21") %>

the result being - Hello World!

Note:
Some people have asked me how to decode those "+"s, which are nothing but encoded spaces in the URL. Although Request.QueryString("your_key") should remove the +s, here is the code that will replace +s with spaces. This is nothing but a modified Listing 5.

Listing 6
<script language=JavaScript RUNAT=SERVER>
// This function decodes the any string
// that's been encoded using URL encoding technique
function URLDecode(psEncodeString)
{
  // Create a regular expression to search all +s in the string
  var lsRegExp = /\+/g;
  // Return the decoded string
  return unescape(String(psEncodeString).replace(lsRegExp, " "));
}
</script>