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A query string (sometimes also known as a search string) is an optional part of a URL that goes after the file or directory name, and begins with a question (?) mark. Query strings are used to pass variables from page to page on a web server, thereby maintaining state on a stateless server.

For example, everything2 uses query strings to pass information on the current node, last visited node, etc... from one page to another.
They look something like this:

  ?node_id=282&lastnode_id=796891

Most web-enabled programming languages have methods for obtaining the query string from a URL.

In ASP, this is accomplished through the Request.QueryString collection. Variables in the query string can be referenced through Request.QueryString(variable_name). Example (in VBScript) using the query string detailed above:

<%
  Response.Write Request.QueryString("node_id") & "<br>"
  Response.Write Request.QueryString("lastnode_id")
%>
would result in the following output to a browser:
  282
  796891

In JavaScript, retrieving the query string is done with the window object's location.search property. For example:

<script language="javascript">
  document.write window.location.search;
<script>
would result in the following output to a browser:
  ?node_id=282&lastnode_id=796891

In Java, query string access is implemented in the java.net.URL class's getQuery() method. The method returns a String object containing the query string.

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