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The advanced expression engine supports expressions for evaluating and 
retrieving data from XML and JavaScript Object Notation (JSON) files. This 
enables you to find specific nodes in an XML or JSON document, determine if a 
node exists in the file, locate nodes in XML contexts (for example, nodes that 
have specific parents or a specific attribute with a given value), and return the 
contents of such nodes. Additionally, you can use XPath expressions in rewrite 
expressions.

The advanced expression implementation for XPath comprises an advanced 
expression prefix (such as “HTTP.REQ.BODY”) that designates XML text and 
the XPATH operator that takes the XPath expression as its argument. 

JSON files are either a collection of name/value pairs or an ordered list of values. 
You can use the XPATH_JSON operator, which takes an XPath expression as its 
argument, to process JSON files.

text.XPATH(xpathex)

Operate on an XML file and return a Boolean 
value. 

For example, the following expression returns 
a Boolean TRUE if a node called “creator” 
exists under the node “Book” within the first 
1000 bytes of the XML file.

HTTP.REQ.BODY(1000).
XPATH(xp%boolean(//Book/creator)%) 

Parameters: 

xpathex - XPath Boolean expression 

text.XPATH(xpathex)

Operate on an XML file and return a value of 
data type “double.” 

For example, the following expression 
converts the string “36” (a price value) to a 
value of data type “double” if the string is in 
the first 1000 bytes of the XML file:

HTTP.REQ.BODY(1000).
XPATH(xp%number(/Book/price)%) 

Parameters: 

xpathex - XPath numeric expression