64 Citrix NetScaler Policy Configuration and Reference Guide
About Text Expressions
You can configure various expressions for working with text that flows through
the NetScaler. Following are some examples of how you can parse text using an
advanced expression:
• Determine that a particular HTTP header exists.
For example, you may want to identify HTTP requests that contains a
particular Accept-Language header for the purpose of directing the request
to a particular server.
• Determine that a particular HTTP URL contains a particular string.
For example, you may want to block requests for particular URLs. Note
that the string can occur at the beginning, middle, or end of another string.
• Identify a POST request that is directed to a particular application.
For example, you may want to identify all POST requests that are directed
to a database application for the purpose of refreshing cached application
data.
Note that there are specialized tools for viewing the data stream for HTTP
requests and responses. For example, you can download a Firefox Web browser
plug-in that displays HTTP request and response headers from the following
URL:
https://addons.mozilla.org/en-US/firefox/addon/3829
The following plug-in displays headers, query strings, POST data, and other
information:
https://addons.mozilla.org/en-US/firefox/addon/6647
After downloading these plug-ins, they are accessible from the Firefox Tools
menu.

About Operations on Text

A text-based expression consists of at least one prefix to identify an element of
data and usually (although not always) an operation on that prefix. Text-based
operations can apply to any part of a request or a response. Basic operations on
text include various types of string matches.
For example, the following expression compares a header value with a string:
http.req.header("myHeader").contains("some-text")
Following expressions are examples of matching a file type in a request:
http.req.url.suffix.contains("jpeg")
http.req.url.suffix.eq("jpeg")