From MathWeb

XPath is a W3C-specified expression language for addressing portions of an XML document, or for computing values (strings, numbers, or boolean values) based on the content of an XML document.

Implementations

• Mozilla/JavaScript: Introduction to using XPath in JavaScript (using DOM Level 3 XPath)
• Java: The Java XPath API

Resources

• Specifications:
  • XPath 1.0
  • XPath 2.0