XPath is a non-XML language for identifying particular parts of XML documents. XPath lets you write expressions that refer to, for example, the first person element in a document, the seventh child element of the third person element, the ID attribute of the first person element whose contents are the string "Fred Jones", all xml-stylesheet processing instructions in the document's prolog, and so forth. XPath indicates nodes by position, relative position, type, content, and several other criteria. XSLT uses XPath expressions to match and select particular elements in the input document for copying into the output document or further processing. XPointer uses XPath expressions to identify the particular point in or part of an XML document to which an XLink links. The W3C XML Schema Language uses XPath expressions to define uniqueness and identity constraints. XForms relies on XPath to bind form controls to instance data, express constraints on user -entered values, and calculate values that depend on other values.
XPath expressions can also represent numbers , strings, or Booleans. This lets XSLT stylesheets carry out simple arithmetic for purposes such as numbering and cross-referencing figures, tables, and equations. String manipulation in XPath lets XSLT perform tasks such as making the title of a chapter uppercase in a headline or extracting the last two digits from a year.