XPath query language

XPath is a query language for XML. XML is structured data where the document is organized as a tree. The relative position of an element with respect to the root is called a Path and there's only one path from that element to the root.The selection criteria for nodes is called predicate. The different ways of slicing a tree or the line to follow is called Axes and these can be for example parent, child or self with child being the default. Paths can be nested inside predicates and predicates can be nested inside paths. Queries are expressed as full or partial paths with selection. They are also expressed in short forms. The position in the xml tree at which the next processing should take place is tracked with a context node.  Nodes can have attributes, namespace and text. Elements positions are 1-based and in document order.
The expressions to denote the path usually describe a starting point, the context node, a line of search if not the child, and other absolute or relative paths. Paths have steps from one level to another and can include a mix or nesting of predicates and grouping via parameters. Query results are returned in document order and XPath does not modify the nodes.  Standard operators such as union and standard functions such as count or id, sum, and startswith are available to use with the query. XPath queries can return all elements in a document based on path-steps as given in the Path expressions.Queries can return attributes that begin anywhere as long as the name is matched. Queries can use wild cards say to denote all children of any element matching a given path. Queries can evaluate selection conditions where the text is compared to a constant. Queries can evaluate conditions based on attributes within the elements.