.. -*- mode: rst -*-

====================
Write your own lexer
====================

If a lexer for your favorite language is missing in the Pygments package, you can
easily write your own and extend Pygments.

All you need can be found inside the `pygments.lexer` module. As you can read in
the `API documentation <api.txt>`_, a lexer is a class that is initialized with
some keyword arguments (the lexer options) and that provides a
`get_tokens_unprocessed()` method which is given a string or unicode object with
the data to parse.

The `get_tokens_unprocessed()` method must return an iterator or iterable
containing tuples in the form ``(index, token, value)``. Normally you don't need
to do this since there are numerous base lexers you can subclass.


RegexLexer
==========

A very powerful (but quite easy to use) lexer is the `RegexLexer`. This lexer
base class allows you to define lexing rules in terms of *regular expressions*
for different *states*.

States are groups of regular expressions that are matched against the input
string at the *current position*. If one of these expressions matches, a
corresponding action is performed (normally yielding a token with a specific
type), the current position is set to where the last match ended and the
matching process continues with the first regex of the current state.

Lexer states are kept in a state stack: each time a new state is entered, the
new state is pushed onto the stack.  The most basic lexers (like the
`DiffLexer`) just need one state.

Each state is defined as a list of tuples in the form (`regex`, `action`,
`new_state`) where the last item is optional.  In the most basic form, `action`
is a token type (like `Name.Builtin`).  That means: When `regex` matches, emit a
token with the match text and type `tokentype` and push `new_state` on the state
stack.  If the new state is ``'#pop'``, the topmost state is popped from the
stack instead. (To pop more than one state, use ``'#pop:2'`` and so on.)
``'#push'`` is a synonym for pushing the current state on the
stack.

The following example shows the `DiffLexer` from the builtin lexers. Note that
it contains some additional attributes `name`, `aliases` and `filenames` which
aren't required for a lexer. They are used by the builtin lexer lookup
functions.

.. sourcecode:: python

    from pygments.lexer import RegexLexer
    from pygments.token import *

    class DiffLexer(RegexLexer):
        name = 'Diff'
        aliases = ['diff']
        filenames = ['*.diff']

        tokens = {
            'root': [
                (r' .*\n', Text),
                (r'\+.*\n', Generic.Inserted),
                (r'-.*\n', Generic.Deleted),
                (r'@.*\n', Generic.Subheading),
                (r'Index.*\n', Generic.Heading),
                (r'=.*\n', Generic.Heading),
                (r'.*\n', Text),
            ]
        }

As you can see this lexer only uses one state.  When the lexer starts scanning
the text, it first checks if the current character is a space. If this is true
it scans everything until newline and returns the parsed data as `Text` token.

If this rule doesn't match, it checks if the current char is a plus sign.  And
so on.

If no rule matches at the current position, the current char is emitted as an
`Error` token that indicates a parsing error, and the position is increased by
1.


Regex Flags
===========

You can either define regex flags in the regex (``r'(?x)foo bar'``) or by adding
a `flags` attribute to your lexer class. If no attribute is defined, it defaults
to `re.MULTILINE`. For more informations about regular expression flags see the
`regular expressions`_ help page in the python documentation.

.. _regular expressions: http://docs.python.org/lib/re-syntax.html


Scanning multiple tokens at once
================================

Here is a more complex lexer that highlights INI files. INI files consist of
sections, comments and key = value pairs:

.. sourcecode:: python

    from pygments.lexer import RegexLexer, bygroups
    from pygments.token import *

    class IniLexer(RegexLexer):
        name = 'INI'
        aliases = ['ini', 'cfg']
        filenames = ['*.ini', '*.cfg']

        tokens = {
            'root': [
                (r'\s+', Text),
                (r';.*?$', Comment),
                (r'\[.*?\]$', Keyword),
                (r'(.*?)(\s*)(=)(\s*)(.*?)$',
                 bygroups(Name.Attribute, Text, Operator, Text, String))
            ]
        }

The lexer first looks for whitespace, comments and section names. And later it
looks for a line that looks like a key, value pair, separated by an ``'='``
sign, and optional whitespace.

The `bygroups` helper makes sure that each group is yielded with a different
token type. First the `Name.Attribute` token, then a `Text` token for the
optional whitespace, after that a `Operator` token for the equals sign. Then a
`Text` token for the whitespace again. The rest of the line is returned as
`String`.

Note that for this to work, every part of the match must be inside a capturing
group (a ``(...)``), and there must not be any nested capturing groups.  If you
nevertheless need a group, use a non-capturing group defined using this syntax:
``r'(?:some|words|here)'`` (note the ``?:`` after the beginning parenthesis).

If you find yourself needing a capturing group inside the regex which
shouldn't be part of the output but is used in the regular expressions for
backreferencing (eg: ``r'(<(foo|bar)>)(.*?)(</\2>)'``), you can pass `None`
to the bygroups function and it will skip that group will be skipped in the
output.


Changing states
===============

Many lexers need multiple states to work as expected. For example, some
languages allow multiline comments to be nested. Since this is a recursive
pattern it's impossible to lex just using regular expressions.

Here is the solution:

.. sourcecode:: python

    from pygments.lexer import RegexLexer
    from pygments.token import *

    class ExampleLexer(RegexLexer):
        name = 'Example Lexer with states'

        tokens = {
            'root': [
                (r'[^/]+', Text),
                (r'/\*', Comment.Multiline, 'comment'),
                (r'//.*?$', Comment.Singleline),
                (r'/', Text)
            ],
            'comment': [
                (r'[^*/]', Comment.Multiline),
                (r'/\*', Comment.Multiline, '#push'),
                (r'\*/', Comment.Multiline, '#pop'),
                (r'[*/]', Comment.Multiline)
            ]
        }

This lexer starts lexing in the ``'root'`` state. It tries to match as much as
possible until it finds a slash (``'/'``). If the next character after the slash
is a star (``'*'``) the `RegexLexer` sends those two characters to the output
stream marked as `Comment.Multiline` and continues parsing with the rules
defined in the ``'comment'`` state.

If there wasn't a star after the slash, the `RegexLexer` checks if it's a
singleline comment (eg: followed by a second slash). If this also wasn't the
case it must be a single slash (the separate regex for a single slash must also
be given, else the slash would be marked as an error token).

Inside the ``'comment'`` state, we do the same thing again. Scan until the lexer
finds a star or slash. If it's the opening of a multiline comment, push the
``'comment'`` state on the stack and continue scanning, again in the
``'comment'`` state.  Else, check if it's the end of the multiline comment. If
yes, pop one state from the stack.

Note: If you pop from an empty stack you'll get an `IndexError`. (There is an
easy way to prevent this from happening: don't ``'#pop'`` in the root state).

If the `RegexLexer` encounters a newline that is flagged as an error token, the
stack is emptied and the lexer continues scanning in the ``'root'`` state. This
helps producing error-tolerant highlighting for erroneous input, e.g. when a
single-line string is not closed.


Advanced state tricks
=====================

There are a few more things you can do with states:

- You can push multiple states onto the stack if you give a tuple instead of a
  simple string as the third item in a rule tuple. For example, if you want to
  match a comment containing a directive, something like::

      /* <processing directive>    rest of comment */

  you can use this rule:

  .. sourcecode:: python

      tokens = {
          'root': [
              (r'/\* <', Comment, ('comment', 'directive')),
              ...
          ],
          'directive': [
              (r'[^>]*', Comment.Directive),
              (r'>', Comment, '#pop'),
          ],
          'comment': [
              (r'[^*]+', Comment),
              (r'\*/', Comment, '#pop'),
              (r'\*', Comment),
          ]
      }

  When this encounters the above sample, first ``'comment'`` and ``'directive'``
  are pushed onto the stack, then the lexer continues in the directive state
  until it finds the closing ``>``, then it continues in the comment state until
  the closing ``*/``. Then, both states are popped from the stack again and
  lexing continues in the root state.

  *New in Pygments 0.9:* The tuple can contain the special ``'#push'`` and
  ``'#pop'`` (but not ``'#pop:n'``) directives.


- You can include the rules of a state in the definition of another.  This is
  done by using `include` from `pygments.lexer`:

  .. sourcecode:: python

      from pygments.lexer import RegexLexer, bygroups, include
      from pygments.token import *

      class ExampleLexer(RegexLexer):
          tokens = {
              'comments': [
                  (r'/\*.*?\*/', Comment),
                  (r'//.*?\n', Comment),
              ],
              'root': [
                  include('comments'),
                  (r'(function )(\w+)( {)',
                   bygroups(Keyword, Name, Keyword), 'function'),
                  (r'.', Text),
              ],
              'function': [
                  (r'[^}/]+', Text),
                  include('comments'),
                  (r'/', Text),
                  (r'}', Keyword, '#pop'),
              ]
          }

  This is a hypothetical lexer for a language that consist of functions and
  comments. Because comments can occur at toplevel and in functions, we need
  rules for comments in both states. As you can see, the `include` helper saves
  repeating rules that occur more than once (in this example, the state
  ``'comment'`` will never be entered by the lexer, as it's only there to be
  included in ``'root'`` and ``'function'``).


- Sometimes, you may want to "combine" a state from existing ones.  This is
  possible with the `combine` helper from `pygments.lexer`.

  If you, instead of a new state, write ``combined('state1', 'state2')`` as the
  third item of a rule tuple, a new anonymous state will be formed from state1
  and state2 and if the rule matches, the lexer will enter this state.

  This is not used very often, but can be helpful in some cases, such as the
  `PythonLexer`'s string literal processing.

- If you want your lexer to start lexing in a different state you can modify
  the stack by overloading the `get_tokens_unprocessed()` method:

  .. sourcecode:: python

      from pygments.lexer import RegexLexer

      class MyLexer(RegexLexer):
          tokens = {...}

          def get_tokens_unprocessed(self, text):
              stack = ['root', 'otherstate']
              for item in RegexLexer.get_tokens_unprocessed(text, stack):
                  yield item

  Some lexers like the `PhpLexer` use this to make the leading ``<?php``
  preprocessor comments optional. Note that you can crash the lexer easily
  by putting values into the stack that don't exist in the token map. Also
  removing ``'root'`` from the stack can result in strange errors!

- An empty regex at the end of a state list, combined with ``'#pop'``, can
  act as a return point from a state that doesn't have a clear end marker.


Using multiple lexers
=====================

Using multiple lexers for the same input can be tricky. One of the easiest
combination techniques is shown here: You can replace the token type entry in a
rule tuple (the second item) with a lexer class. The matched text will then be
lexed with that lexer, and the resulting tokens will be yielded.

For example, look at this stripped-down HTML lexer:

.. sourcecode:: python

    from pygments.lexer import RegexLexer, bygroups, using
    from pygments.token import *
    from pygments.lexers.web import JavascriptLexer

    class HtmlLexer(RegexLexer):
        name = 'HTML'
        aliases = ['html']
        filenames = ['*.html', '*.htm']

        flags = re.IGNORECASE | re.DOTALL
        tokens = {
            'root': [
                ('[^<&]+', Text),
                ('&.*?;', Name.Entity),
                (r'<\s*script\s*', Name.Tag, ('script-content', 'tag')),
                (r'<\s*[a-zA-Z0-9:]+', Name.Tag, 'tag'),
                (r'<\s*/\s*[a-zA-Z0-9:]+\s*>', Name.Tag),
            ],
            'script-content': [
                (r'(.+?)(<\s*/\s*script\s*>)',
                 bygroups(using(JavascriptLexer), Name.Tag),
                 '#pop'),
            ]
        }

Here the content of a ``<script>`` tag is passed to a newly created instance of
a `JavascriptLexer` and not processed by the `HtmlLexer`. This is done using the
`using` helper that takes the other lexer class as its parameter.

Note the combination of `bygroups` and `using`. This makes sure that the content
up to the ``</script>`` end tag is processed by the `JavascriptLexer`, while the
end tag is yielded as a normal token with the `Name.Tag` type.

As an additional goodie, if the lexer class is replaced by `this` (imported from
`pygments.lexer`), the "other" lexer will be the current one (because you cannot
refer to the current class within the code that runs at class definition time).

Also note the ``(r'<\s*script\s*', Name.Tag, ('script-content', 'tag'))`` rule.
Here, two states are pushed onto the state stack, ``'script-content'`` and
``'tag'``.  That means that first ``'tag'`` is processed, which will parse
attributes and the closing ``>``, then the ``'tag'`` state is popped and the
next state on top of the stack will be ``'script-content'``.

The `using()` helper has a special keyword argument, `state`, which works as
follows: if given, the lexer to use initially is not in the ``"root"`` state,
but in the state given by this argument. This *only* works with a `RegexLexer`.

Any other keywords arguments passed to `using()` are added to the keyword
arguments used to create the lexer.


Delegating Lexer
================

Another approach for nested lexers is the `DelegatingLexer` which is for
example used for the template engine lexers. It takes two lexers as
arguments on initialisation: a `root_lexer` and a `language_lexer`.

The input is processed as follows: First, the whole text is lexed with the
`language_lexer`. All tokens yielded with a type of ``Other`` are then
concatenated and given to the `root_lexer`. The language tokens of the
`language_lexer` are then inserted into the `root_lexer`'s token stream
at the appropriate positions.

.. sourcecode:: python

    from pygments.lexer import DelegatingLexer
    from pygments.lexers.web import HtmlLexer, PhpLexer

    class HtmlPhpLexer(DelegatingLexer):
        def __init__(self, **options):
            super(HtmlPhpLexer, self).__init__(HtmlLexer, PhpLexer, **options)

This procedure ensures that e.g. HTML with template tags in it is highlighted
correctly even if the template tags are put into HTML tags or attributes.

If you want to change the needle token ``Other`` to something else, you can
give the lexer another token type as the third parameter:

.. sourcecode:: python

    DelegatingLexer.__init__(MyLexer, OtherLexer, Text, **options)


Callbacks
=========

Sometimes the grammar of a language is so complex that a lexer would be unable
to parse it just by using regular expressions and stacks.

For this, the `RegexLexer` allows callbacks to be given in rule tuples, instead
of token types (`bygroups` and `using` are nothing else but preimplemented
callbacks). The callback must be a function taking two arguments:

* the lexer itself
* the match object for the last matched rule

The callback must then return an iterable of (or simply yield) ``(index,
tokentype, value)`` tuples, which are then just passed through by
`get_tokens_unprocessed()`. The ``index`` here is the position of the token in
the input string, ``tokentype`` is the normal token type (like `Name.Builtin`),
and ``value`` the associated part of the input string.

You can see an example here:

.. sourcecode:: python

    from pygments.lexer import RegexLexer
    from pygments.token import Generic

    class HypotheticLexer(RegexLexer):

        def headline_callback(lexer, match):
            equal_signs = match.group(1)
            text = match.group(2)
            yield match.start(), Generic.Headline, equal_signs + text + equal_signs

        tokens = {
            'root': [
                (r'(=+)(.*?)(\1)', headline_callback)
            ]
        }

If the regex for the `headline_callback` matches, the function is called with the
match object. Note that after the callback is done, processing continues
normally, that is, after the end of the previous match. The callback has no
possibility to influence the position.

There are not really any simple examples for lexer callbacks, but you can see
them in action e.g. in the `compiled.py`_ source code in the `CLexer` and
`JavaLexer` classes.

.. _compiled.py: http://bitbucket.org/birkenfeld/pygments-main/src/tip/pygments/lexers/compiled.py


The ExtendedRegexLexer class
============================

The `RegexLexer`, even with callbacks, unfortunately isn't powerful enough for
the funky syntax rules of some languages that will go unnamed, such as Ruby.

But fear not; even then you don't have to abandon the regular expression
approach. For Pygments has a subclass of `RegexLexer`, the `ExtendedRegexLexer`.
All features known from RegexLexers are available here too, and the tokens are
specified in exactly the same way, *except* for one detail:

The `get_tokens_unprocessed()` method holds its internal state data not as local
variables, but in an instance of the `pygments.lexer.LexerContext` class, and
that instance is passed to callbacks as a third argument. This means that you
can modify the lexer state in callbacks.

The `LexerContext` class has the following members:

* `text` -- the input text
* `pos` -- the current starting position that is used for matching regexes
* `stack` -- a list containing the state stack
* `end` -- the maximum position to which regexes are matched, this defaults to
  the length of `text`

Additionally, the `get_tokens_unprocessed()` method can be given a
`LexerContext` instead of a string and will then process this context instead of
creating a new one for the string argument.

Note that because you can set the current position to anything in the callback,
it won't be automatically be set by the caller after the callback is finished.
For example, this is how the hypothetical lexer above would be written with the
`ExtendedRegexLexer`:

.. sourcecode:: python

    from pygments.lexer import ExtendedRegexLexer
    from pygments.token import Generic

    class ExHypotheticLexer(ExtendedRegexLexer):

        def headline_callback(lexer, match, ctx):
            equal_signs = match.group(1)
            text = match.group(2)
            yield match.start(), Generic.Headline, equal_signs + text + equal_signs
            ctx.pos = match.end()

        tokens = {
            'root': [
                (r'(=+)(.*?)(\1)', headline_callback)
            ]
        }

This might sound confusing (and it can really be). But it is needed, and for an
example look at the Ruby lexer in `agile.py`_.

.. _agile.py: https://bitbucket.org/birkenfeld/pygments-main/src/tip/pygments/lexers/agile.py


Filtering Token Streams
=======================

Some languages ship a lot of builtin functions (for example PHP). The total
amount of those functions differs from system to system because not everybody
has every extension installed. In the case of PHP there are over 3000 builtin
functions. That's an incredible huge amount of functions, much more than you
can put into a regular expression.

But because only `Name` tokens can be function names it's solvable by overriding
the ``get_tokens_unprocessed()`` method. The following lexer subclasses the
`PythonLexer` so that it highlights some additional names as pseudo keywords:

.. sourcecode:: python

    from pygments.lexers.agile import PythonLexer
    from pygments.token import Name, Keyword

    class MyPythonLexer(PythonLexer):
        EXTRA_KEYWORDS = ['foo', 'bar', 'foobar', 'barfoo', 'spam', 'eggs']

        def get_tokens_unprocessed(self, text):
            for index, token, value in PythonLexer.get_tokens_unprocessed(self, text):
                if token is Name and value in self.EXTRA_KEYWORDS:
                    yield index, Keyword.Pseudo, value
                else:
                    yield index, token, value

The `PhpLexer` and `LuaLexer` use this method to resolve builtin functions.

**Note** Do not confuse this with the `filter`_ system.

.. _filter: filters.txt
