.. _xpath-tutorial:

XPath Tutorial
==============

In this tutorial, you will be given a gentle introduction to 
`XPath <https://en.wikipedia.org/wiki/XPath>`_, a query language that can be 
used to select arbitrary parts of `HTML <https://en.wikipedia.org/wiki/HTML>`_
documents in calibre. XPath is a widely
used standard, and googling it will yield a ton of information. This tutorial, 
however, focuses on using XPath for ebook related tasks like finding chapter 
headings in an unstructured HTML document.

.. contents:: Contents
  :depth: 1
  :local: 

Selecting by tagname
----------------------------------------

The simplest form of selection is to select tags by name. For example, 
suppose you want to select all the ``<h2>`` tags in a document. The XPath
query for this is simply::

    //h:h2        (Selects all <h2> tags)

The prefix `//` means *search at any level of the document*. Now suppose you
want to search for ``<span>`` tags that are inside ``<a>`` tags. That can be
achieved with::

    //h:a/h:span    (Selects <span> tags inside <a> tags)

If you want to search for tags at a particular level in the document, change
the prefix::

    /h:body/h:div/h:p (Selects <p> tags that are children of <div> tags that are
                 children of the <body> tag)

This will match only ``<p>A very short ebook to demonstrate the use of XPath.</p>`` 
in the :ref:`sample_ebook` but not any of the other ``<p>`` tags. The ``h:`` prefix
in the above examples is needed to match XHTML tags. This is because internally,
calibre represents all content as XHTML. In XHTML tags have a *namespace*, and
``h:`` is the namespace prefix for HTML tags.

Now suppose you want to select both ``<h1>`` and ``<h2>`` tags. To do that,
we need a XPath construct called *predicate*. A :dfn:`predicate` is simply 
a test that is used to select tags. Tests can be arbitrarily powerful and as
this tutorial progresses, you will see more powerful examples. A predicate
is created by enclosing the test expression in square brackets::

//*[name()='h1' or name()='h2']

There are several new features in this XPath expression. The first is the use
of the wildcard ``*``. It means *match any tag*. Now look at the test expression
``name()='h1' or name()='h2'``. :term:`name()` is an example of a *built-in function*.
It simply evaluates to the name of the tag. So by using it, we can select tags
whose names are either `h1` or `h2`. Note that the :term:`name()` function 
ignores namespaces so that there is no need for the ``h:`` prefix.
XPath has several useful built-in functions. A few more will be introduced in this tutorial.

Selecting by attributes
-----------------------

To select tags based on their attributes, the use of predicates is required::

    //*[@style]              (Select all tags that have a style attribute)
    //*[@class="chapter"]    (Select all tags that have class="chapter")
    //h:h1[@class="bookTitle"] (Select all h1 tags that have class="bookTitle")

Here, the ``@`` operator refers to the attributes of the tag. You can use some 
of the `XPath built-in functions`_ to perform more sophisticated
matching on attribute values.


Selecting by tag content
------------------------

Using XPath, you can even select tags based on the text they contain. The best way to do this is
to use the power of *regular expressions* via the built-in function :term:`re:test()`::

    //h:h2[re:test(., 'chapter|section', 'i')] (Selects <h2> tags that contain the words chapter or 
                                              section)

Here the ``.`` operator refers to the contents of the tag, just as the ``@`` operator referred
to its attributes.

.. _sample_ebook :

Sample ebook
------------

.. literalinclude:: xpath.xhtml
    :language: html

XPath built-in functions
------------------------

.. glossary::

    name()
        The name of the current tag.

    contains()
        ``contains(s1, s2)`` returns `true` if s1 contains s2.

    re:test()
        ``re:test(src, pattern, flags)`` returns `true` if the string `src` matches the
        regular expression `pattern`. A particularly useful flag is ``i``, it makes matching
        case insensitive. A good primer on the syntax for regular expressions can be found
        at `regexp syntax <https://docs.python.org/2.7/library/re.html>`_

