.. -*- mode: rst -*-

.. highlight:: python

====================
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 :mod:`pygments.lexer` module.  As you can
read in the :doc:`API documentation <api>`, a lexer is a class that is
initialized with some keyword arguments (the lexer options) and that provides a
:meth:`.get_tokens_unprocessed()` method which is given a string or unicode
object with the data to lex.

The :meth:`.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 base lexers that do most of the work and that
you can subclass.

RegexLexer
==========

The lexer base class used by almost all of Pygments' lexers is the
:class:`RegexLexer <pygments.lexer.RegexLexer>`.  This 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 (such as yielding a token with a specific
type, or changing state), 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 on a 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 a second time the current state on top of
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. ::

    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 data as a `Text` token (which
is the "no special highlighting" 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 lexing error, and the position is increased by
one.


Using a lexer
=============

The easiest way to use a new lexer is to use Pygments' support for loading
the lexer from a file relative to your current directory.

First, change the name of your lexer class to CustomLexer:

.. code-block:: python

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

    class CustomLexer(RegexLexer):
        """All your lexer code goes here!"""

Then you can load and test the lexer from the command line with the additional
flag ``-x``:

.. code-block:: console

    $ python -m pygments -x -l your_lexer_file.py <inputfile>

To specify a class name other than CustomLexer, append it with a colon:

.. code-block:: console

    $ python -m pygments -x -l your_lexer.py:SomeLexer <inputfile>

Or, using the Python API:

.. code-block:: python

    # For a lexer named CustomLexer
    your_lexer = load_lexer_from_file(filename, **options)

    # For a lexer named MyNewLexer
    your_named_lexer = load_lexer_from_file(filename, "MyNewLexer", **options)

When loading custom lexers and formatters, be extremely careful to use only
trusted files; Pygments will perform the equivalent of ``eval`` on them.

If you only want to use your lexer with the Pygments API, you can import and
instantiate the lexer yourself, then pass it to :func:`pygments.highlight`.

Use the ``-f`` flag to select a different output format than terminal
escape sequences. The :class:`.HtmlFormatter` helps
you with debugging your lexer. You can use the ``debug_token_types`` option
to display the token types assigned to each part of your input file:

.. code-block:: console

    $ python -m pygments -x -f html -Ofull,debug_token_types -l your_lexer.py:SomeLexer <inputfile>

Hover over each token to see the token type displayed as a tooltip.

If your lexer would be useful to other people, we would love if you
contributed it to Pygments.  See :doc:`contributing` for advice.


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

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

.. _regular expressions: https://docs.python.org/library/re.html#regular-expression-syntax


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

So far, the `action` element in the rule tuple of regex, action and state has
been a single token type.  Now we look at the first of several other possible
values.

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

    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.  Later it
looks for a line that looks like a key, value pair, separated by an ``'='``
sign, and optional whitespace.

The `bygroups` helper yields each capturing group in the regex 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:
``(?: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 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 a lexer that recognizes C++ style comments (multi-line with ``/* */``
and single-line with ``//`` until end of line)::

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

    class CppCommentLexer(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 an asterisk (``'*'``) the `RegexLexer` sends those two characters to the
output stream marked as `Comment.Multiline` and continues lexing with the rules
defined in the ``'comment'`` state.

If there wasn't an asterisk after the slash, the `RegexLexer` checks if it's a
Singleline comment (i.e. followed by a second slash).  If this also wasn't the
case it must be a single slash, which is not a comment starter (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
can help 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:

  .. code-block:: text

      /* <processing directive>    rest of comment */

  you can use this rule::

      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.

  .. versionadded:: 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`::

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

      class ExampleLexer(RegexLexer):
          tokens = {
              'comments': [
                  (r'(?s)/\*.*?\*/', Comment),
                  (r'//.*?\n', Comment),
              ],
              'root': [
                  include('comments'),
                  (r'(function)( )(\w+)( )({)',
                   bygroups(Keyword, Whitespace, Name, Whitespace, Punctuation), 'function'),
                  (r'.*\n', Text),
              ],
              'function': [
                  (r'[^}/]+', Text),
                  include('comments'),
                  (r'/', Text),
                  (r'\}', Punctuation, '#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 `combined` 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 overriding the `get_tokens_unprocessed()` method::

      from pygments.lexer import RegexLexer

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

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

- In some lexers, a state should be popped if anything is encountered that isn't
  matched by a rule in the state.  You could use an empty regex at the end of
  the state list, but Pygments provides a more obvious way of spelling that:
  ``default('#pop')`` is equivalent to ``('', Text, '#pop')``.

  .. versionadded:: 2.0


Subclassing lexers derived from RegexLexer
==========================================

.. versionadded:: 1.6

Sometimes multiple languages are very similar, but should still be lexed by
different lexer classes.

When subclassing a lexer derived from RegexLexer, the ``tokens`` dictionaries
defined in the parent and child class are merged.  For example::

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

      class BaseLexer(RegexLexer):
          tokens = {
              'root': [
                  ('[a-z]+', Name),
                  (r'/\*', Comment, 'comment'),
                  ('"', String, 'string'),
                  (r'\s+', Text),
              ],
              'string': [
                  ('[^"]+', String),
                  ('"', String, '#pop'),
              ],
              'comment': [
                  ...
              ],
          }

      class DerivedLexer(BaseLexer):
          tokens = {
              'root': [
                  ('[0-9]+', Number),
                  inherit,
              ],
              'string': [
                  (r'[^"\\]+', String),
                  (r'\\.', String.Escape),
                  ('"', String, '#pop'),
              ],
          }

The `BaseLexer` defines two states, lexing names and strings.  The
`DerivedLexer` defines its own tokens dictionary, which extends the definitions
of the base lexer:

* The "root" state has an additional rule and then the special object `inherit`,
  which tells Pygments to insert the token definitions of the parent class at
  that point.

* The "string" state is replaced entirely, since there is not `inherit` rule.

* The "comment" state is inherited entirely.


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 action entry in a rule
tuple 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::

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

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 lex
attributes and the closing ``>``, then the ``'tag'`` state is popped and the
next state on top of the stack will be ``'script-content'``.

Since you cannot refer to the class currently being defined, use `this`
(imported from `pygments.lexer`) to refer to the current lexer class, i.e.
``using(this)``.  This construct may seem unnecessary, but this is often the
most obvious way of lexing arbitrary syntax between fixed delimiters without
introducing deeply nested states.

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 does not work with advanced
`RegexLexer` subclasses such as `ExtendedRegexLexer` (see below).

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 the special 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. ::

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

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

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


Callbacks
=========

Sometimes the grammar of a language is so complex that a lexer would be unable
to process 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::

    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 `SMLLexer` class in `ml.py`_.

.. _ml.py: https://github.com/pygments/pygments/blob/master/pygments/lexers/ml.py


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

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

But fear not; even then you don't have to abandon the regular expression
approach: 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`::

    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 `ruby.py`_.

.. _ruby.py: https://github.com/pygments/pygments/blob/master/pygments/lexers/ruby.py


Handling Lists of Keywords
==========================

For a relatively short list (hundreds) you can construct an optimized regular
expression directly using ``words()`` (longer lists, see next section).  This
function handles a few things for you automatically, including escaping
metacharacters and Python's first-match rather than longest-match in
alternations.  Feel free to put the lists themselves in
``pygments/lexers/_$lang_builtins.py`` (see examples there), and generated by
code if possible.

An example of using ``words()`` is something like::

    from pygments.lexer import RegexLexer, words, Name

    class MyLexer(RegexLexer):

        tokens = {
            'root': [
                (words(('else', 'elseif'), suffix=r'\b'), Name.Builtin),
                (r'\w+', Name),
            ],
        }

As you can see, you can add ``prefix`` and ``suffix`` parts to the constructed
regex.


Modifying 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 incredibly huge amount of functions, much more than you
want to put into a regular expression.

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

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

    class MyPythonLexer(PythonLexer):
        EXTRA_KEYWORDS = set(('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.



.. _lexer-pitfalls:

Common pitfalls and best practices
==================================

Regular expressions are ubiquitous in Pygments lexers.  We have
written this section to warn about a few common mistakes you might do
when using them. There are also some tips on making your lexers easier
to read and review. You are asked to read this section if you want to
contribute a new lexer, but you might find it useful in any case.


* When writing rules, try to merge simple rules. For instance, combine::

     (r"\(", token.Punctuation),
     (r"\)", token.Punctuation),
     (r"\[", token.Punctuation),
     (r"\]", token.Punctuation),
     ("{", token.Punctuation),
     ("}", token.Punctuation),

  into::

   (r"[\(\)\[\]{}]", token.Punctuation)


* Be careful with ``.*``. This matches greedily as much as it can. For instance,
  a rule like ``@.*@`` will match the whole string ``@first@ second @third@``,
  instead of matching ``@first@`` and ``@second@``. You can use ``@.*?@`` in
  this case to stop early. The ``?`` tries to match *as few times* as possible.

* Beware of so-called "catastrophic backtracking".  As a first example, consider
  the regular expression ``(A+)*B``.  This is equivalent to ``A*B`` regarding
  what it matches, but *non*-matches will take very long.  This is because
  of the way the regular expression engine works.  Suppose you feed it 50
  'A's, and a 'C' at the end.  It first matches the 'A's greedily in ``A+``,
  but finds that it cannot match the end since 'B' is not the same as 'C'.
  Then it backtracks, removing one 'A' from the first ``A+`` and trying to
  match the rest as another ``(A+)*``.  This fails again, so it backtracks
  further left in the input string, etc.  In effect, it tries all combinations

  .. code-block:: text

     (AAAAAAAAAAAAAAAAA)
     (AAAAAAAAAAAAAAAA)(A)
     (AAAAAAAAAAAAAAA)(AA)
     (AAAAAAAAAAAAAAA)(A)(A)
     (AAAAAAAAAAAAAA)(AAA)
     (AAAAAAAAAAAAAA)(AA)(A)
     ...

  Thus, the matching has exponential complexity.  In a lexer, the
  effect is that Pygments will seemingly hang when parsing invalid
  input. ::

     >>> import re
     >>> re.match('(A+)*B', 'A'*50 + 'C') # hangs

  As a more subtle and real-life example, here is a badly written
  regular expression to match strings::

     r'"(\\?.)*?"'

  If the ending quote is missing, the regular expression engine will
  find that it cannot match at the end, and try to backtrack with less
  matches in the ``*?``.  When it finds a backslash, as it has already
  tried the possibility ``\\.``, it tries ``.`` (recognizing it as a
  simple character without meaning), which leads to the same
  exponential backtracking problem if there are lots of backslashes in
  the (invalid) input string.  A good way to write this would be
  ``r'"([^\\]|\\.)*?"'``, where the inner group can only match in one
  way.  Better yet is to use a dedicated state, which not only
  sidesteps the issue without headaches, but allows you to highlight
  string escapes. ::

     'root': [
         ...,
         (r'"', String, 'string'),
         ...
     ],
     'string': [
         (r'\\.', String.Escape),
         (r'"', String, '#pop'),
         (r'[^\\"]+', String),
     ]

* When writing rules for patterns such as comments or strings, match as many
  characters as possible in each token.  This is an example of what *not* to
  do::

     'comment': [
         (r'\*/', Comment.Multiline, '#pop'),
         (r'.', Comment.Multiline),
     ]

  This generates one token per character in the comment, which slows
  down the lexing process, and also makes the raw token output (and in
  particular the test output) hard to read.  Do this instead::

     'comment': [
         (r'\*/', Comment.Multiline, '#pop'),
         (r'[^*]+', Comment.Multiline),
         (r'\*', Comment.Multiline),
     ]
