You may find A Brief Introduction to XPath 2.0 helpful as a starting point for working with XPath expressions in mashups. You can also create your own custom XPath functions to use in expressions. See Defining Custom XPath Functions for information.

In addition, expressions can be dynamic in mashups. You can use Dynamic Mashup Expressions directly in XPath. Or, use Dynamic XPath or Other Syntax in Mashups to pass dynamic portions of XPath as parameters. Finally, see Using XQuery Expressions in Construction Statements for information on dynamic expressions that you use to help construct the mashup result or intermediate variables.