Added optional Lua lexer support.

This support is disabled by default and must be enabled via compile-time option.

1 files changed, 1865 insertions, 0 deletions

diff --git a/lexlua/lexer.lua b/lexlua/lexer.lua
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+-- Copyright 2006-2018 Mitchell mitchell.att.foicica.com. See License.txt.
+
+local M = {}
+
+--[=[ This comment is for LuaDoc.
+---
+-- Lexes Scintilla documents and source code with Lua and LPeg.
+--
+-- ## Writing Lua Lexers
+--
+-- Lexers highlight the syntax of source code. Scintilla (the editing component
+-- behind [Textadept][]) traditionally uses static, compiled C++ lexers which
+-- are notoriously difficult to create and/or extend. On the other hand, Lua
+-- makes it easy to to rapidly create new lexers, extend existing ones, and
+-- embed lexers within one another. Lua lexers tend to be more readable than C++
+-- lexers too.
+--
+-- Lexers are Parsing Expression Grammars, or PEGs, composed with the Lua
+-- [LPeg library][]. The following table comes from the LPeg documentation and
+-- summarizes all you need to know about constructing basic LPeg patterns. This
+-- module provides convenience functions for creating and working with other
+-- more advanced patterns and concepts.
+--
+-- Operator             | Description
+-- ---------------------|------------
+-- `lpeg.P(string)`     | Matches `string` literally.
+-- `lpeg.P(`_`n`_`)`    | Matches exactly _`n`_ characters.
+-- `lpeg.S(string)`     | Matches any character in set `string`.
+-- `lpeg.R("`_`xy`_`")` | Matches any character between range `x` and `y`.
+-- `patt^`_`n`_         | Matches at least _`n`_ repetitions of `patt`.
+-- `patt^-`_`n`_        | Matches at most _`n`_ repetitions of `patt`.
+-- `patt1 * patt2`      | Matches `patt1` followed by `patt2`.
+-- `patt1 + patt2`      | Matches `patt1` or `patt2` (ordered choice).
+-- `patt1 - patt2`      | Matches `patt1` if `patt2` does not match.
+-- `-patt`              | Equivalent to `("" - patt)`.
+-- `#patt`              | Matches `patt` but consumes no input.
+--
+-- The first part of this document deals with rapidly constructing a simple
+-- lexer. The next part deals with more advanced techniques, such as custom
+-- coloring and embedding lexers within one another. Following that is a
+-- discussion about code folding, or being able to tell Scintilla which code
+-- blocks are "foldable" (temporarily hideable from view). After that are
+-- instructions on how to use Lua lexers with the aforementioned Textadept
+-- editor. Finally there are comments on lexer performance and limitations.
+--
+-- [LPeg library]: http://www.inf.puc-rio.br/~roberto/lpeg/lpeg.html
+-- [Textadept]: http://foicica.com/textadept
+--
+-- ### Lexer Basics
+--
+-- The *lexers/* directory contains all lexers, including your new one. Before
+-- attempting to write one from scratch though, first determine if your
+-- programming language is similar to any of the 100+ languages supported. If
+-- so, you may be able to copy and modify that lexer, saving some time and
+-- effort. The filename of your lexer should be the name of your programming
+-- language in lower case followed by a *.lua* extension. For example, a new Lua
+-- lexer has the name *lua.lua*.
+--
+-- Note: Try to refrain from using one-character language names like "c", "d",
+-- or "r". For example, Lua lexers for those language names are named "ansi_c",
+-- "dmd", and "rstats", respectively.
+--
+-- #### New Lexer Template
+--
+-- There is a *lexers/template.txt* file that contains a simple template for a
+-- new lexer. Feel free to use it, replacing the '?'s with the name of your
+-- lexer. Consider this snippet from the template:
+--
+--     -- ? LPeg lexer.
+--
+--     local lexer = require('lexer')
+--     local token, word_match = lexer.token, lexer.word_match
+--     local P, R, S = lpeg.P, lpeg.R, lpeg.S
+--
+--     local lex = lexer.new('?')
+--
+--     -- Whitespace.
+--     local ws = token(lexer.WHITESPACE, lexer.space^1)
+--     lex:add_rule('whitespace', ws)
+--
+--     [...]
+--
+--     return lex
+--
+-- The first 3 lines of code simply define often used convenience variables. The
+-- fourth and last lines [define](#lexer.new) and return the lexer object
+-- Scintilla uses; they are very important and must be part of every lexer. The
+-- fifth line defines something called a "token", an essential building block of
+-- lexers. You will learn about tokens shortly. The sixth line defines a lexer
+-- grammar rule, which you will learn about later, as well as token styles. (Be
+-- aware that it is common practice to combine these two lines for short rules.)
+-- Note, however, the `local` prefix in front of variables, which is needed
+-- so-as not to affect Lua's global environment. All in all, this is a minimal,
+-- working lexer that you can build on.
+--
+-- #### Tokens
+--
+-- Take a moment to think about your programming language's structure. What kind
+-- of key elements does it have? In the template shown earlier, one predefined
+-- element all languages have is whitespace. Your language probably also has
+-- elements like comments, strings, and keywords. Lexers refer to these elements
+-- as "tokens". Tokens are the fundamental "building blocks" of lexers. Lexers
+-- break down source code into tokens for coloring, which results in the syntax
+-- highlighting familiar to you. It is up to you how specific your lexer is when
+-- it comes to tokens. Perhaps only distinguishing between keywords and
+-- identifiers is necessary, or maybe recognizing constants and built-in
+-- functions, methods, or libraries is desirable. The Lua lexer, for example,
+-- defines 11 tokens: whitespace, keywords, built-in functions, constants,
+-- built-in libraries, identifiers, strings, comments, numbers, labels, and
+-- operators. Even though constants, built-in functions, and built-in libraries
+-- are subsets of identifiers, Lua programmers find it helpful for the lexer to
+-- distinguish between them all. It is perfectly acceptable to just recognize
+-- keywords and identifiers.
+--
+-- In a lexer, tokens consist of a token name and an LPeg pattern that matches a
+-- sequence of characters recognized as an instance of that token. Create tokens
+-- using the [`lexer.token()`]() function. Let us examine the "whitespace" token
+-- defined in the template shown earlier:
+--
+--     local ws = token(lexer.WHITESPACE, lexer.space^1)
+--
+-- At first glance, the first argument does not appear to be a string name and
+-- the second argument does not appear to be an LPeg pattern. Perhaps you
+-- expected something like:
+--
+--     local ws = token('whitespace', S('\t\v\f\n\r ')^1)
+--
+-- The `lexer` module actually provides a convenient list of common token names
+-- and common LPeg patterns for you to use. Token names include
+-- [`lexer.DEFAULT`](), [`lexer.WHITESPACE`](), [`lexer.COMMENT`](),
+-- [`lexer.STRING`](), [`lexer.NUMBER`](), [`lexer.KEYWORD`](),
+-- [`lexer.IDENTIFIER`](), [`lexer.OPERATOR`](), [`lexer.ERROR`](),
+-- [`lexer.PREPROCESSOR`](), [`lexer.CONSTANT`](), [`lexer.VARIABLE`](),
+-- [`lexer.FUNCTION`](), [`lexer.CLASS`](), [`lexer.TYPE`](), [`lexer.LABEL`](),
+-- [`lexer.REGEX`](), and [`lexer.EMBEDDED`](). Patterns include
+-- [`lexer.any`](), [`lexer.ascii`](), [`lexer.extend`](), [`lexer.alpha`](),
+-- [`lexer.digit`](), [`lexer.alnum`](), [`lexer.lower`](), [`lexer.upper`](),
+-- [`lexer.xdigit`](), [`lexer.cntrl`](), [`lexer.graph`](), [`lexer.print`](),
+-- [`lexer.punct`](), [`lexer.space`](), [`lexer.newline`](),
+-- [`lexer.nonnewline`](), [`lexer.nonnewline_esc`](), [`lexer.dec_num`](),
+-- [`lexer.hex_num`](), [`lexer.oct_num`](), [`lexer.integer`](),
+-- [`lexer.float`](), and [`lexer.word`](). You may use your own token names if
+-- none of the above fit your language, but an advantage to using predefined
+-- token names is that your lexer's tokens will inherit the universal syntax
+-- highlighting color theme used by your text editor.
+--
+-- ##### Example Tokens
+--
+-- So, how might you define other tokens like keywords, comments, and strings?
+-- Here are some examples.
+--
+-- **Keywords**
+--
+-- Instead of matching _n_ keywords with _n_ `P('keyword_`_`n`_`')` ordered
+-- choices, use another convenience function: [`lexer.word_match()`](). It is
+-- much easier and more efficient to write word matches like:
+--
+--     local keyword = token(lexer.KEYWORD, lexer.word_match[[
+--       keyword_1 keyword_2 ... keyword_n
+--     ]])
+--
+--     local case_insensitive_keyword = token(lexer.KEYWORD, lexer.word_match([[
+--       KEYWORD_1 keyword_2 ... KEYword_n
+--     ]], true))
+--
+--     local hyphened_keyword = token(lexer.KEYWORD, lexer.word_match[[
+--       keyword-1 keyword-2 ... keyword-n
+--     ]])
+--
+-- In order to more easily separate or categorize keyword sets, you can use Lua
+-- line comments within keyword strings. Such comments will be ignored. For
+-- example:
+--
+--     local keyword = token(lexer.KEYWORD, lexer.word_match[[
+--       -- Version 1 keywords.
+--       keyword_11, keyword_12 ... keyword_1n
+--       -- Version 2 keywords.
+--       keyword_21, keyword_22 ... keyword_2n
+--       ...
+--       -- Version N keywords.
+--       keyword_m1, keyword_m2 ... keyword_mn
+--     ]])
+--
+-- **Comments**
+--
+-- Line-style comments with a prefix character(s) are easy to express with LPeg:
+--
+--     local shell_comment = token(lexer.COMMENT, '#' * lexer.nonnewline^0)
+--     local c_line_comment = token(lexer.COMMENT,
+--                                  '//' * lexer.nonnewline_esc^0)
+--
+-- The comments above start with a '#' or "//" and go to the end of the line.
+-- The second comment recognizes the next line also as a comment if the current
+-- line ends with a '\' escape character.
+--
+-- C-style "block" comments with a start and end delimiter are also easy to
+-- express:
+--
+--     local c_comment = token(lexer.COMMENT,
+--                             '/*' * (lexer.any - '*/')^0 * P('*/')^-1)
+--
+-- This comment starts with a "/\*" sequence and contains anything up to and
+-- including an ending "\*/" sequence. The ending "\*/" is optional so the lexer
+-- can recognize unfinished comments as comments and highlight them properly.
+--
+-- **Strings**
+--
+-- It is tempting to think that a string is not much different from the block
+-- comment shown above in that both have start and end delimiters:
+--
+--     local dq_str = '"' * (lexer.any - '"')^0 * P('"')^-1
+--     local sq_str = "'" * (lexer.any - "'")^0 * P("'")^-1
+--     local simple_string = token(lexer.STRING, dq_str + sq_str)
+--
+-- However, most programming languages allow escape sequences in strings such
+-- that a sequence like "\\"" in a double-quoted string indicates that the
+-- '"' is not the end of the string. The above token incorrectly matches
+-- such a string. Instead, use the [`lexer.delimited_range()`]() convenience
+-- function.
+--
+--     local dq_str = lexer.delimited_range('"')
+--     local sq_str = lexer.delimited_range("'")
+--     local string = token(lexer.STRING, dq_str + sq_str)
+--
+-- In this case, the lexer treats '\' as an escape character in a string
+-- sequence.
+--
+-- **Numbers**
+--
+-- Most programming languages have the same format for integer and float tokens,
+-- so it might be as simple as using a couple of predefined LPeg patterns:
+--
+--     local number = token(lexer.NUMBER, lexer.float + lexer.integer)
+--
+-- However, some languages allow postfix characters on integers.
+--
+--     local integer = P('-')^-1 * (lexer.dec_num * S('lL')^-1)
+--     local number = token(lexer.NUMBER, lexer.float + lexer.hex_num + integer)
+--
+-- Your language may need other tweaks, but it is up to you how fine-grained you
+-- want your highlighting to be. After all, you are not writing a compiler or
+-- interpreter!
+--
+-- #### Rules
+--
+-- Programming languages have grammars, which specify valid token structure. For
+-- example, comments usually cannot appear within a string. Grammars consist of
+-- rules, which are simply combinations of tokens. Recall from the lexer
+-- template the [`lexer.add_rule()`]() call, which adds a rule to the lexer's
+-- grammar:
+--
+--     lex:add_rule('whitespace', ws)
+--
+-- Each rule has an associated name, but rule names are completely arbitrary and
+-- serve only to identify and distinguish between different rules. Rule order is
+-- important: if text does not match the first rule added to the grammar, the
+-- lexer tries to match the second rule added, and so on. Right now this lexer
+-- simply matches whitespace tokens under a rule named "whitespace".
+--
+-- To illustrate the importance of rule order, here is an example of a
+-- simplified Lua lexer:
+--
+--     lex:add_rule('whitespace', token(lexer.WHITESPACE, ...))
+--     lex:add_rule('keyword', token(lexer.KEYWORD, ...))
+--     lex:add_rule('identifier', token(lexer.IDENTIFIER, ...))
+--     lex:add_rule('string', token(lexer.STRING, ...))
+--     lex:add_rule('comment', token(lexer.COMMENT, ...))
+--     lex:add_rule('number', token(lexer.NUMBER, ...))
+--     lex:add_rule('label', token(lexer.LABEL, ...))
+--     lex:add_rule('operator', token(lexer.OPERATOR, ...))
+--
+-- Note how identifiers come after keywords. In Lua, as with most programming
+-- languages, the characters allowed in keywords and identifiers are in the same
+-- set (alphanumerics plus underscores). If the lexer added the "identifier"
+-- rule before the "keyword" rule, all keywords would match identifiers and thus
+-- incorrectly highlight as identifiers instead of keywords. The same idea
+-- applies to function, constant, etc. tokens that you may want to distinguish
+-- between: their rules should come before identifiers.
+--
+-- So what about text that does not match any rules? For example in Lua, the '!'
+-- character is meaningless outside a string or comment. Normally the lexer
+-- skips over such text. If instead you want to highlight these "syntax errors",
+-- add an additional end rule:
+--
+--     lex:add_rule('whitespace', ws)
+--     ...
+--     lex:add_rule('error', token(lexer.ERROR, lexer.any))
+--
+-- This identifies and highlights any character not matched by an existing
+-- rule as a `lexer.ERROR` token.
+--
+-- Even though the rules defined in the examples above contain a single token,
+-- rules may consist of multiple tokens. For example, a rule for an HTML tag
+-- could consist of a tag token followed by an arbitrary number of attribute
+-- tokens, allowing the lexer to highlight all tokens separately. That rule
+-- might look something like this:
+--
+--     lex:add_rule('tag', tag_start * (ws * attributes)^0 * tag_end^-1)
+--
+-- Note however that lexers with complex rules like these are more prone to lose
+-- track of their state, especially if they span multiple lines.
+--
+-- #### Summary
+--
+-- Lexers primarily consist of tokens and grammar rules. At your disposal are a
+-- number of convenience patterns and functions for rapidly creating a lexer. If
+-- you choose to use predefined token names for your tokens, you do not have to
+-- define how the lexer highlights them. The tokens will inherit the default
+-- syntax highlighting color theme your editor uses.
+--
+-- ### Advanced Techniques
+--
+-- #### Styles and Styling
+--
+-- The most basic form of syntax highlighting is assigning different colors to
+-- different tokens. Instead of highlighting with just colors, Scintilla allows
+-- for more rich highlighting, or "styling", with different fonts, font sizes,
+-- font attributes, and foreground and background colors, just to name a few.
+-- The unit of this rich highlighting is called a "style". Styles are simply
+-- strings of comma-separated property settings. By default, lexers associate
+-- predefined token names like `lexer.WHITESPACE`, `lexer.COMMENT`,
+-- `lexer.STRING`, etc. with particular styles as part of a universal color
+-- theme. These predefined styles include [`lexer.STYLE_CLASS`](),
+-- [`lexer.STYLE_COMMENT`](), [`lexer.STYLE_CONSTANT`](),
+-- [`lexer.STYLE_ERROR`](), [`lexer.STYLE_EMBEDDED`](),
+-- [`lexer.STYLE_FUNCTION`](), [`lexer.STYLE_IDENTIFIER`](),
+-- [`lexer.STYLE_KEYWORD`](), [`lexer.STYLE_LABEL`](), [`lexer.STYLE_NUMBER`](),
+-- [`lexer.STYLE_OPERATOR`](), [`lexer.STYLE_PREPROCESSOR`](),
+-- [`lexer.STYLE_REGEX`](), [`lexer.STYLE_STRING`](), [`lexer.STYLE_TYPE`](),
+-- [`lexer.STYLE_VARIABLE`](), and [`lexer.STYLE_WHITESPACE`](). Like with
+-- predefined token names and LPeg patterns, you may define your own styles. At
+-- their core, styles are just strings, so you may create new ones and/or modify
+-- existing ones. Each style consists of the following comma-separated settings:
+--
+-- Setting        | Description
+-- ---------------|------------
+-- font:_name_    | The name of the font the style uses.
+-- size:_int_     | The size of the font the style uses.
+-- [not]bold      | Whether or not the font face is bold.
+-- weight:_int_   | The weight or boldness of a font, between 1 and 999.
+-- [not]italics   | Whether or not the font face is italic.
+-- [not]underlined| Whether or not the font face is underlined.
+-- fore:_color_   | The foreground color of the font face.
+-- back:_color_   | The background color of the font face.
+-- [not]eolfilled | Does the background color extend to the end of the line?
+-- case:_char_    | The case of the font ('u': upper, 'l': lower, 'm': normal).
+-- [not]visible   | Whether or not the text is visible.
+-- [not]changeable| Whether the text is changeable or read-only.
+--
+-- Specify font colors in either "#RRGGBB" format, "0xBBGGRR" format, or the
+-- decimal equivalent of the latter. As with token names, LPeg patterns, and
+-- styles, there is a set of predefined color names, but they vary depending on
+-- the current color theme in use. Therefore, it is generally not a good idea to
+-- manually define colors within styles in your lexer since they might not fit
+-- into a user's chosen color theme. Try to refrain from even using predefined
+-- colors in a style because that color may be theme-specific. Instead, the best
+-- practice is to either use predefined styles or derive new color-agnostic
+-- styles from predefined ones. For example, Lua "longstring" tokens use the
+-- existing `lexer.STYLE_STRING` style instead of defining a new one.
+--
+-- ##### Example Styles
+--
+-- Defining styles is pretty straightforward. An empty style that inherits the
+-- default theme settings is simply an empty string:
+--
+--     local style_nothing = ''
+--
+-- A similar style but with a bold font face looks like this:
+--
+--     local style_bold = 'bold'
+--
+-- If you want the same style, but also with an italic font face, define the new
+-- style in terms of the old one:
+--
+--     local style_bold_italic = style_bold..',italics'
+--
+-- This allows you to derive new styles from predefined ones without having to
+-- rewrite them. This operation leaves the old style unchanged. Thus if you
+-- had a "static variable" token whose style you wanted to base off of
+-- `lexer.STYLE_VARIABLE`, it would probably look like:
+--
+--     local style_static_var = lexer.STYLE_VARIABLE..',italics'
+--
+-- The color theme files in the *lexers/themes/* folder give more examples of
+-- style definitions.
+--
+-- #### Token Styles
+--
+-- Lexers use the [`lexer.add_style()`]() function to assign styles to
+-- particular tokens. Recall the token definition and from the lexer template:
+--
+--     local ws = token(lexer.WHITESPACE, lexer.space^1)
+--     lex:add_rule('whitespace', ws)
+--
+-- Why is a style not assigned to the `lexer.WHITESPACE` token? As mentioned
+-- earlier, lexers automatically associate tokens that use predefined token
+-- names with a particular style. Only tokens with custom token names need
+-- manual style associations. As an example, consider a custom whitespace token:
+--
+--     local ws = token('custom_whitespace', lexer.space^1)
+--
+-- Assigning a style to this token looks like:
+--
+--     lex:add_style('custom_whitespace', lexer.STYLE_WHITESPACE)
+--
+-- Do not confuse token names with rule names. They are completely different
+-- entities. In the example above, the lexer associates the "custom_whitespace"
+-- token with the existing style for `lexer.WHITESPACE` tokens. If instead you
+-- prefer to color the background of whitespace a shade of grey, it might look
+-- like:
+--
+--     local custom_style = lexer.STYLE_WHITESPACE..',back:$(color.grey)'
+--     lex:add_style('custom_whitespace', custom_style)
+--
+-- Notice that the lexer peforms Scintilla-style "$()" property expansion. You
+-- may also use "%()". Remember to refrain from assigning specific colors in
+-- styles, but in this case, all user color themes probably define the
+-- "color.grey" property.
+--
+-- #### Line Lexers
+--
+-- By default, lexers match the arbitrary chunks of text passed to them by
+-- Scintilla. These chunks may be a full document, only the visible part of a
+-- document, or even just portions of lines. Some lexers need to match whole
+-- lines. For example, a lexer for the output of a file "diff" needs to know if
+-- the line started with a '+' or '-' and then style the entire line
+-- accordingly. To indicate that your lexer matches by line, create the lexer
+-- with an extra parameter:
+--
+--     local lex = lexer.new('?', {lex_by_line = true})
+--
+-- Now the input text for the lexer is a single line at a time. Keep in mind
+-- that line lexers do not have the ability to look ahead at subsequent lines.
+--
+-- #### Embedded Lexers
+--
+-- Lexers embed within one another very easily, requiring minimal effort. In the
+-- following sections, the lexer being embedded is called the "child" lexer and
+-- the lexer a child is being embedded in is called the "parent". For example,
+-- consider an HTML lexer and a CSS lexer. Either lexer stands alone for styling
+-- their respective HTML and CSS files. However, CSS can be embedded inside
+-- HTML. In this specific case, the CSS lexer is the "child" lexer with the HTML
+-- lexer being the "parent". Now consider an HTML lexer and a PHP lexer. This
+-- sounds a lot like the case with CSS, but there is a subtle difference: PHP
+-- _embeds itself into_ HTML while CSS is _embedded in_ HTML. This fundamental
+-- difference results in two types of embedded lexers: a parent lexer that
+-- embeds other child lexers in it (like HTML embedding CSS), and a child lexer
+-- that embeds itself into a parent lexer (like PHP embedding itself in HTML).
+--
+-- ##### Parent Lexer
+--
+-- Before embedding a child lexer into a parent lexer, the parent lexer needs to
+-- load the child lexer. This is done with the [`lexer.load()`]() function. For
+-- example, loading the CSS lexer within the HTML lexer looks like:
+--
+--     local css = lexer.load('css')
+--
+-- The next part of the embedding process is telling the parent lexer when to
+-- switch over to the child lexer and when to switch back. The lexer refers to
+-- these indications as the "start rule" and "end rule", respectively, and are
+-- just LPeg patterns. Continuing with the HTML/CSS example, the transition from
+-- HTML to CSS is when the lexer encounters a "style" tag with a "type"
+-- attribute whose value is "text/css":
+--
+--     local css_tag = P('<style') * P(function(input, index)
+--       if input:find('^[^>]+type="text/css"', index) then
+--         return index
+--       end
+--     end)
+--
+-- This pattern looks for the beginning of a "style" tag and searches its
+-- attribute list for the text "`type="text/css"`". (In this simplified example,
+-- the Lua pattern does not consider whitespace between the '=' nor does it
+-- consider that using single quotes is valid.) If there is a match, the
+-- functional pattern returns a value instead of `nil`. In this case, the value
+-- returned does not matter because we ultimately want to style the "style" tag
+-- as an HTML tag, so the actual start rule looks like this:
+--
+--     local css_start_rule = #css_tag * tag
+--
+-- Now that the parent knows when to switch to the child, it needs to know when
+-- to switch back. In the case of HTML/CSS, the switch back occurs when the
+-- lexer encounters an ending "style" tag, though the lexer should still style
+-- the tag as an HTML tag:
+--
+--     local css_end_rule = #P('</style>') * tag
+--
+-- Once the parent loads the child lexer and defines the child's start and end
+-- rules, it embeds the child with the [`lexer.embed()`]() function:
+--
+--     lex:embed(css, css_start_rule, css_end_rule)
+--
+-- ##### Child Lexer
+--
+-- The process for instructing a child lexer to embed itself into a parent is
+-- very similar to embedding a child into a parent: first, load the parent lexer
+-- into the child lexer with the [`lexer.load()`]() function and then create
+-- start and end rules for the child lexer. However, in this case, call
+-- [`lexer.embed()`]() with switched arguments. For example, in the PHP lexer:
+--
+--     local html = lexer.load('html')
+--     local php_start_rule = token('php_tag', '<?php ')
+--     local php_end_rule = token('php_tag', '?>')
+--     lex:add_style('php_tag', lexer.STYLE_EMBEDDED)
+--     html:embed(lex, php_start_rule, php_end_rule)
+--
+-- #### Lexers with Complex State
+--
+-- A vast majority of lexers are not stateful and can operate on any chunk of
+-- text in a document. However, there may be rare cases where a lexer does need
+-- to keep track of some sort of persistent state. Rather than using `lpeg.P`
+-- function patterns that set state variables, it is recommended to make use of
+-- Scintilla's built-in, per-line state integers via [`lexer.line_state`](). It
+-- was designed to accommodate up to 32 bit flags for tracking state.
+-- [`lexer.line_from_position()`]() will return the line for any position given
+-- to an `lpeg.P` function pattern. (Any positions derived from that position
+-- argument will also work.)
+--
+-- Writing stateful lexers is beyond the scope of this document.
+--
+-- ### Code Folding
+--
+-- When reading source code, it is occasionally helpful to temporarily hide
+-- blocks of code like functions, classes, comments, etc. This is the concept of
+-- "folding". In many Scintilla-based editors, such as Textadept, little
+-- indicators in the editor margins appear next to code that can be folded at
+-- places called "fold points". When the user clicks an indicator, the editor
+-- hides the code associated with the indicator until the user clicks the
+-- indicator again. The lexer specifies these fold points and what code exactly
+-- to fold.
+--
+-- The fold points for most languages occur on keywords or character sequences.
+-- Examples of fold keywords are "if" and "end" in Lua and examples of fold
+-- character sequences are '{', '}', "/\*", and "\*/" in C for code block and
+-- comment delimiters, respectively. However, these fold points cannot occur
+-- just anywhere. For example, lexers should not recognize fold keywords that
+-- appear within strings or comments. The [`lexer.add_fold_point()`]() function
+-- allows you to conveniently define fold points with such granularity. For
+-- example, consider C:
+--
+--     lex:add_fold_point(lexer.OPERATOR, '{', '}')
+--     lex:add_fold_point(lexer.COMMENT, '/*', '*/')
+--
+-- The first assignment states that any '{' or '}' that the lexer recognized as
+-- an `lexer.OPERATOR` token is a fold point. Likewise, the second assignment
+-- states that any "/\*" or "\*/" that the lexer recognizes as part of a
+-- `lexer.COMMENT` token is a fold point. The lexer does not consider any
+-- occurrences of these characters outside their defined tokens (such as in a
+-- string) as fold points. How do you specify fold keywords? Here is an example
+-- for Lua:
+--
+--     lex:add_fold_point(lexer.KEYWORD, 'if', 'end')
+--     lex:add_fold_point(lexer.KEYWORD, 'do', 'end')
+--     lex:add_fold_point(lexer.KEYWORD, 'function', 'end')
+--     lex:add_fold_point(lexer.KEYWORD, 'repeat', 'until')
+--
+-- If your lexer has case-insensitive keywords as fold points, simply add a
+-- `case_insensitive_fold_points = true` option to [`lexer.new()`](), and
+-- specify keywords in lower case.
+--
+-- If your lexer needs to do some additional processing in order to determine if
+-- a token is a fold point, pass a function that returns an integer to
+-- `lex:add_fold_point()`. Returning `1` indicates the token is a beginning fold
+-- point and returning `-1` indicates the token is an ending fold point.
+-- Returning `0` indicates the token is not a fold point. For example:
+--
+--     local function fold_strange_token(text, pos, line, s, symbol)
+--       if ... then
+--         return 1 -- beginning fold point
+--       elseif ... then
+--         return -1 -- ending fold point
+--       end
+--       return 0
+--     end
+--
+--     lex:add_fold_point('strange_token', '|', fold_strange_token)
+--
+-- Any time the lexer encounters a '|' that is a "strange_token", it calls the
+-- `fold_strange_token` function to determine if '|' is a fold point. The lexer
+-- calls these functions with the following arguments: the text to identify fold
+-- points in, the beginning position of the current line in the text to fold,
+-- the current line's text, the position in the current line the fold point text
+-- starts at, and the fold point text itself.
+--
+-- #### Fold by Indentation
+--
+-- Some languages have significant whitespace and/or no delimiters that indicate
+-- fold points. If your lexer falls into this category and you would like to
+-- mark fold points based on changes in indentation, create the lexer with a
+-- `fold_by_indentation = true` option:
+--
+--     local lex = lexer.new('?', {fold_by_indentation = true})
+--
+-- ### Using Lexers
+--
+-- #### Textadept
+--
+-- Put your lexer in your *~/.textadept/lexers/* directory so you do not
+-- overwrite it when upgrading Textadept. Also, lexers in this directory
+-- override default lexers. Thus, Textadept loads a user *lua* lexer instead of
+-- the default *lua* lexer. This is convenient for tweaking a default lexer to
+-- your liking. Then add a [file type][] for your lexer if necessary.
+--
+-- [file type]: textadept.file_types.html
+--
+-- ### Migrating Legacy Lexers
+--
+-- Legacy lexers are of the form:
+--
+--     local l = require('lexer')
+--     local token, word_match = l.token, l.word_match
+--     local P, R, S = lpeg.P, lpeg.R, lpeg.S
+--
+--     local M = {_NAME = '?'}
+--
+--     [... token and pattern definitions ...]
+--
+--     M._rules = {
+--       {'rule', pattern},
+--       [...]
+--     }
+--
+--     M._tokenstyles = {
+--       'token' = 'style',
+--       [...]
+--     }
+--
+--     M._foldsymbols = {
+--       _patterns = {...},
+--       ['token'] = {['start'] = 1, ['end'] = -1},
+--       [...]
+--     }
+--
+--     return M
+--
+-- While such legacy lexers will be handled just fine without any changes, it is
+-- recommended that you migrate yours. The migration process is fairly
+-- straightforward:
+--
+-- 1. Replace all instances of `l` with `lexer`, as it's better practice and
+--    results in less confusion.
+-- 2. Replace `local M = {_NAME = '?'}` with `local lex = lexer.new('?')`, where
+--    `?` is the name of your legacy lexer. At the end of the lexer, change
+--    `return M` to `return lex`.
+-- 3. Instead of defining rules towards the end of your lexer, define your rules
+--    as you define your tokens and patterns using
+--    [`lex:add_rule()`](#lexer.add_rule).
+-- 4. Similarly, any custom token names should have their styles immediately
+--    defined using [`lex:add_style()`](#lexer.add_style).
+-- 5. Convert any table arguments passed to [`lexer.word_match()`]() to a
+--    space-separated string of words.
+-- 6. Replace any calls to `lexer.embed(M, child, ...)` and
+--    `lexer.embed(parent, M, ...)` with
+--    [`lex:embed`](#lexer.embed)`(child, ...)` and `parent:embed(lex, ...)`,
+--    respectively.
+-- 7. Define fold points with simple calls to
+--    [`lex:add_fold_point()`](#lexer.add_fold_point). No need to mess with Lua
+--    patterns anymore.
+-- 8. Any legacy lexer options such as `M._FOLDBYINDENTATION`, `M._LEXBYLINE`,
+--    `M._lexer`, etc. should be added as table options to [`lexer.new()`]().
+-- 9. Any external lexer rule fetching and/or modifications via `lexer._RULES`
+--    should be changed to use [`lexer.get_rule()`]() and
+--    [`lexer.modify_rule()`]().
+--
+-- As an example, consider the following sample legacy lexer:
+--
+--     local l = require('lexer')
+--     local token, word_match = l.token, l.word_match
+--     local P, R, S = lpeg.P, lpeg.R, lpeg.S
+--
+--     local M = {_NAME = 'legacy'}
+--
+--     local ws = token(l.WHITESPACE, l.space^1)
+--     local comment = token(l.COMMENT, '#' * l.nonnewline^0)
+--     local string = token(l.STRING, l.delimited_range('"'))
+--     local number = token(l.NUMBER, l.float + l.integer)
+--     local keyword = token(l.KEYWORD, word_match{'foo', 'bar', 'baz'})
+--     local custom = token('custom', P('quux'))
+--     local identifier = token(l.IDENTIFIER, l.word)
+--     local operator = token(l.OPERATOR, S('+-*/%^=<>,.()[]{}'))
+--
+--     M._rules = {
+--       {'whitespace', ws},
+--       {'keyword', keyword},
+--       {'custom', custom},
+--       {'identifier', identifier},
+--       {'string', string},
+--       {'comment', comment},
+--       {'number', number},
+--       {'operator', operator}
+--     }
+--
+--     M._tokenstyles = {
+--       'custom' = l.STYLE_KEYWORD..',bold'
+--     }
+--
+--     M._foldsymbols = {
+--       _patterns = {'[{}]'},
+--       [l.OPERATOR] = {['{'] = 1, ['}'] = -1}
+--     }
+--
+--     return M
+--
+-- Following the migration steps would yield:
+--
+--     local lexer = require('lexer')
+--     local token, word_match = lexer.token, lexer.word_match
+--     local P, R, S = lpeg.P, lpeg.R, lpeg.S
+--
+--     local lex = lexer.new('legacy')
+--
+--     lex:add_rule('whitespace', token(lexer.WHITESPACE, lexer.space^1))
+--     lex:add_rule('keyword', token(lexer.KEYWORD, word_match[[foo bar baz]]))
+--     lex:add_rule('custom', token('custom', P('quux')))
+--     lex:add_style('custom', lexer.STYLE_KEYWORD..',bold')
+--     lex:add_rule('identifier', token(lexer.IDENTIFIER, lexer.word))
+--     lex:add_rule('string', token(lexer.STRING, lexer.delimited_range('"')))
+--     lex:add_rule('comment', token(lexer.COMMENT, '#' * lexer.nonnewline^0))
+--     lex:add_rule('number', token(lexer.NUMBER, lexer.float + lexer.integer))
+--     lex:add_rule('operator', token(lexer.OPERATOR, S('+-*/%^=<>,.()[]{}')))
+--
+--     lex:add_fold_point(lexer.OPERATOR, '{', '}')
+--
+--     return lex
+--
+-- ### Considerations
+--
+-- #### Performance
+--
+-- There might be some slight overhead when initializing a lexer, but loading a
+-- file from disk into Scintilla is usually more expensive. On modern computer
+-- systems, I see no difference in speed between Lua lexers and Scintilla's C++
+-- ones. Optimize lexers for speed by re-arranging `lexer.add_rule()` calls so
+-- that the most common rules match first. Do keep in mind that order matters
+-- for similar rules.
+--
+-- In some cases, folding may be far more expensive than lexing, particularly
+-- in lexers with a lot of potential fold points. If your lexer is exhibiting
+-- signs of slowness, try disabling folding in your text editor first. If that
+-- speeds things up, you can try reducing the number of fold points you added,
+-- overriding `lexer.fold()` with your own implementation, or simply eliminating
+-- folding support from your lexer.
+--
+-- #### Limitations
+--
+-- Embedded preprocessor languages like PHP cannot completely embed in their
+-- parent languages in that the parent's tokens do not support start and end
+-- rules. This mostly goes unnoticed, but code like
+--
+--     <div id="<?php echo $id; ?>">
+--
+-- will not style correctly.
+--
+-- #### Troubleshooting
+--
+-- Errors in lexers can be tricky to debug. Lexers print Lua errors to
+-- `io.stderr` and `_G.print()` statements to `io.stdout`. Running your editor
+-- from a terminal is the easiest way to see errors as they occur.
+--
+-- #### Risks
+--
+-- Poorly written lexers have the ability to crash Scintilla (and thus its
+-- containing application), so unsaved data might be lost. However, I have only
+-- observed these crashes in early lexer development, when syntax errors or
+-- pattern errors are present. Once the lexer actually starts styling text
+-- (either correctly or incorrectly, it does not matter), I have not observed
+-- any crashes.
+--
+-- #### Acknowledgements
+--
+-- Thanks to Peter Odding for his [lexer post][] on the Lua mailing list
+-- that inspired me, and thanks to Roberto Ierusalimschy for LPeg.
+--
+-- [lexer post]: http://lua-users.org/lists/lua-l/2007-04/msg00116.html
+-- @field path (string)
+--   The path used to search for a lexer to load.
+--   Identical in format to Lua's `package.path` string.
+--   The default value is `package.path`.
+-- @field DEFAULT (string)
+--   The token name for default tokens.
+-- @field WHITESPACE (string)
+--   The token name for whitespace tokens.
+-- @field COMMENT (string)
+--   The token name for comment tokens.
+-- @field STRING (string)
+--   The token name for string tokens.
+-- @field NUMBER (string)
+--   The token name for number tokens.
+-- @field KEYWORD (string)
+--   The token name for keyword tokens.
+-- @field IDENTIFIER (string)
+--   The token name for identifier tokens.
+-- @field OPERATOR (string)
+--   The token name for operator tokens.
+-- @field ERROR (string)
+--   The token name for error tokens.
+-- @field PREPROCESSOR (string)
+--   The token name for preprocessor tokens.
+-- @field CONSTANT (string)
+--   The token name for constant tokens.
+-- @field VARIABLE (string)
+--   The token name for variable tokens.
+-- @field FUNCTION (string)
+--   The token name for function tokens.
+-- @field CLASS (string)
+--   The token name for class tokens.
+-- @field TYPE (string)
+--   The token name for type tokens.
+-- @field LABEL (string)
+--   The token name for label tokens.
+-- @field REGEX (string)
+--   The token name for regex tokens.
+-- @field STYLE_CLASS (string)
+--   The style typically used for class definitions.
+-- @field STYLE_COMMENT (string)
+--   The style typically used for code comments.
+-- @field STYLE_CONSTANT (string)
+--   The style typically used for constants.
+-- @field STYLE_ERROR (string)
+--   The style typically used for erroneous syntax.
+-- @field STYLE_FUNCTION (string)
+--   The style typically used for function definitions.
+-- @field STYLE_KEYWORD (string)
+--   The style typically used for language keywords.
+-- @field STYLE_LABEL (string)
+--   The style typically used for labels.
+-- @field STYLE_NUMBER (string)
+--   The style typically used for numbers.
+-- @field STYLE_OPERATOR (string)
+--   The style typically used for operators.
+-- @field STYLE_REGEX (string)
+--   The style typically used for regular expression strings.
+-- @field STYLE_STRING (string)
+--   The style typically used for strings.
+-- @field STYLE_PREPROCESSOR (string)
+--   The style typically used for preprocessor statements.
+-- @field STYLE_TYPE (string)
+--   The style typically used for static types.
+-- @field STYLE_VARIABLE (string)
+--   The style typically used for variables.
+-- @field STYLE_WHITESPACE (string)
+--   The style typically used for whitespace.
+-- @field STYLE_EMBEDDED (string)
+--   The style typically used for embedded code.
+-- @field STYLE_IDENTIFIER (string)
+--   The style typically used for identifier words.
+-- @field STYLE_DEFAULT (string)
+--   The style all styles are based off of.
+-- @field STYLE_LINENUMBER (string)
+--   The style used for all margins except fold margins.
+-- @field STYLE_BRACELIGHT (string)
+--   The style used for highlighted brace characters.
+-- @field STYLE_BRACEBAD (string)
+--   The style used for unmatched brace characters.
+-- @field STYLE_CONTROLCHAR (string)
+--   The style used for control characters.
+--   Color attributes are ignored.
+-- @field STYLE_INDENTGUIDE (string)
+--   The style used for indentation guides.
+-- @field STYLE_CALLTIP (string)
+--   The style used by call tips if [`buffer.call_tip_use_style`]() is set.
+--   Only the font name, size, and color attributes are used.
+-- @field STYLE_FOLDDISPLAYTEXT (string)
+--   The style used for fold display text.
+-- @field any (pattern)
+--   A pattern that matches any single character.
+-- @field ascii (pattern)
+--   A pattern that matches any ASCII character (codes 0 to 127).
+-- @field extend (pattern)
+--   A pattern that matches any ASCII extended character (codes 0 to 255).
+-- @field alpha (pattern)
+--   A pattern that matches any alphabetic character ('A'-'Z', 'a'-'z').
+-- @field digit (pattern)
+--   A pattern that matches any digit ('0'-'9').
+-- @field alnum (pattern)
+--   A pattern that matches any alphanumeric character ('A'-'Z', 'a'-'z',
+--     '0'-'9').
+-- @field lower (pattern)
+--   A pattern that matches any lower case character ('a'-'z').
+-- @field upper (pattern)
+--   A pattern that matches any upper case character ('A'-'Z').
+-- @field xdigit (pattern)
+--   A pattern that matches any hexadecimal digit ('0'-'9', 'A'-'F', 'a'-'f').
+-- @field cntrl (pattern)
+--   A pattern that matches any control character (ASCII codes 0 to 31).
+-- @field graph (pattern)
+--   A pattern that matches any graphical character ('!' to '~').
+-- @field print (pattern)
+--   A pattern that matches any printable character (' ' to '~').
+-- @field punct (pattern)
+--   A pattern that matches any punctuation character ('!' to '/', ':' to '@',
+--   '[' to ''', '{' to '~').
+-- @field space (pattern)
+--   A pattern that matches any whitespace character ('\t', '\v', '\f', '\n',
+--   '\r', space).
+-- @field newline (pattern)
+--   A pattern that matches any set of end of line characters.
+-- @field nonnewline (pattern)
+--   A pattern that matches any single, non-newline character.
+-- @field nonnewline_esc (pattern)
+--   A pattern that matches any single, non-newline character or any set of end
+--   of line characters escaped with '\'.
+-- @field dec_num (pattern)
+--   A pattern that matches a decimal number.
+-- @field hex_num (pattern)
+--   A pattern that matches a hexadecimal number.
+-- @field oct_num (pattern)
+--   A pattern that matches an octal number.
+-- @field integer (pattern)
+--   A pattern that matches either a decimal, hexadecimal, or octal number.
+-- @field float (pattern)
+--   A pattern that matches a floating point number.
+-- @field word (pattern)
+--   A pattern that matches a typical word. Words begin with a letter or
+--   underscore and consist of alphanumeric and underscore characters.
+-- @field FOLD_BASE (number)
+--   The initial (root) fold level.
+-- @field FOLD_BLANK (number)
+--   Flag indicating that the line is blank.
+-- @field FOLD_HEADER (number)
+--   Flag indicating the line is fold point.
+-- @field fold_level (table, Read-only)
+--   Table of fold level bit-masks for line numbers starting from zero.
+--   Fold level masks are composed of an integer level combined with any of the
+--   following bits:
+--
+--   * `lexer.FOLD_BASE`
+--     The initial fold level.
+--   * `lexer.FOLD_BLANK`
+--     The line is blank.
+--   * `lexer.FOLD_HEADER`
+--     The line is a header, or fold point.
+-- @field indent_amount (table, Read-only)
+--   Table of indentation amounts in character columns, for line numbers
+--   starting from zero.
+-- @field line_state (table)
+--   Table of integer line states for line numbers starting from zero.
+--   Line states can be used by lexers for keeping track of persistent states.
+-- @field property (table)
+--   Map of key-value string pairs.
+-- @field property_expanded (table, Read-only)
+--   Map of key-value string pairs with `$()` and `%()` variable replacement
+--   performed in values.
+-- @field property_int (table, Read-only)
+--   Map of key-value pairs with values interpreted as numbers, or `0` if not
+--   found.
+-- @field style_at (table, Read-only)
+--   Table of style names at positions in the buffer starting from 1.
+module('lexer')]=]
+
+local lpeg = require('lpeg')
+local lpeg_P, lpeg_R, lpeg_S, lpeg_V = lpeg.P, lpeg.R, lpeg.S, lpeg.V
+local lpeg_Ct, lpeg_Cc, lpeg_Cp = lpeg.Ct, lpeg.Cc, lpeg.Cp
+local lpeg_Cmt, lpeg_C = lpeg.Cmt, lpeg.C
+local lpeg_match = lpeg.match
+
+M.path = package.path
+
+if not package.searchpath then
+  -- Searches for the given *name* in the given *path*.
+  -- This is an implementation of Lua 5.2's `package.searchpath()` function for
+  -- Lua 5.1.
+  function package.searchpath(name, path)
+    local tried = {}
+    for part in path:gmatch('[^;]+') do
+      local filename = part:gsub('%?', name)
+      local f = io.open(filename, 'r')
+      if f then
+        f:close()
+        return filename
+      end
+      tried[#tried + 1] = string.format("no file '%s'", filename)
+    end
+    return nil, table.concat(tried, '\n')
+  end
+end
+
+local string_upper = string.upper
+-- Default styles.
+local default = {
+  'nothing', 'whitespace', 'comment', 'string', 'number', 'keyword',
+  'identifier', 'operator', 'error', 'preprocessor', 'constant', 'variable',
+  'function', 'class', 'type', 'label', 'regex', 'embedded'
+}
+for i = 1, #default do
+  local name, upper_name = default[i], string_upper(default[i])
+  M[upper_name], M['STYLE_'..upper_name] = name, '$(style.'..name..')'
+end
+-- Predefined styles.
+local predefined = {
+  'default', 'linenumber', 'bracelight', 'bracebad', 'controlchar',
+  'indentguide', 'calltip', 'folddisplaytext'
+}
+for i = 1, #predefined do
+  local name, upper_name = predefined[i], string_upper(predefined[i])
+  M[upper_name], M['STYLE_'..upper_name] = name, '$(style.'..name..')'
+end
+
+---
+-- Adds pattern *rule* identified by string *id* to the ordered list of rules
+-- for lexer *lexer*.
+-- @param lexer The lexer to add the given rule to.
+-- @param id The id associated with this rule. It does not have to be the same
+--   as the name passed to `token()`.
+-- @param rule The LPeg pattern of the rule.
+-- @see modify_rule
+-- @name add_rule
+function M.add_rule(lexer, id, rule)
+  if lexer._lexer then lexer = lexer._lexer end -- proxy; get true parent
+  if not lexer._RULES then
+    lexer._RULES = {}
+    -- Contains an ordered list (by numerical index) of rule names. This is used
+    -- in conjunction with lexer._RULES for building _TOKENRULE.
+    lexer._RULEORDER = {}
+  end
+  lexer._RULES[id] = rule
+  lexer._RULEORDER[#lexer._RULEORDER + 1] = id
+  lexer:build_grammar()
+end
+
+---
+-- Replaces in lexer *lexer* the existing rule identified by string *id* with
+-- pattern *rule*.
+-- @param lexer The lexer to modify.
+-- @param id The id associated with this rule.
+-- @param rule The LPeg pattern of the rule.
+-- @name modify_rule
+function M.modify_rule(lexer, id, rule)
+  if lexer._lexer then lexer = lexer._lexer end -- proxy; get true parent
+  lexer._RULES[id] = rule
+  lexer:build_grammar()
+end
+
+---
+-- Returns the rule identified by string *id*.
+-- @param lexer The lexer to fetch a rule from.
+-- @param id The id of the rule to fetch.
+-- @return pattern
+-- @name get_rule
+function M.get_rule(lexer, id)
+  if lexer._lexer then lexer = lexer._lexer end -- proxy; get true parent
+  return lexer._RULES[id]
+end
+
+---
+-- Associates string *token_name* in lexer *lexer* with Scintilla style string
+-- *style*.
+-- Style strings are comma-separated property settings. Available property
+-- settings are:
+--
+--   * `font:name`: Font name.
+--   * `size:int`: Font size.
+--   * `bold` or `notbold`: Whether or not the font face is bold.
+--   * `weight:int`: Font weight (between 1 and 999).
+--   * `italics` or `notitalics`: Whether or not the font face is italic.
+--   * `underlined` or `notunderlined`: Whether or not the font face is
+--     underlined.
+--   * `fore:color`: Font face foreground color in "#RRGGBB" or 0xBBGGRR format.
+--   * `back:color`: Font face background color in "#RRGGBB" or 0xBBGGRR format.
+--   * `eolfilled` or `noteolfilled`: Whether or not the background color
+--     extends to the end of the line.
+--   * `case:char`: Font case ('u' for uppercase, 'l' for lowercase, and 'm' for
+--     mixed case).
+--   * `visible` or `notvisible`: Whether or not the text is visible.
+--   * `changeable` or `notchangeable`: Whether or not the text is changeable or
+--     read-only.
+--
+-- Property settings may also contain "$(property.name)" expansions for
+-- properties defined in Scintilla, theme files, etc.
+-- @param lexer The lexer to add a style to.
+-- @param token_name The name of the token to associated with the style.
+-- @param style A style string for Scintilla.
+-- @usage lex:add_style('longstring', lexer.STYLE_STRING)
+-- @usage lex:add_style('deprecated_function', lexer.STYLE_FUNCTION..',italics')
+-- @usage lex:add_style('visible_ws',
+--   lexer.STYLE_WHITESPACE..',back:$(color.grey)')
+-- @name add_style
+function M.add_style(lexer, token_name, style)
+  local num_styles = lexer._numstyles
+  if num_styles == 32 then num_styles = num_styles + 8 end -- skip predefined
+  if num_styles >= 255 then print('Too many styles defined (255 MAX)') end
+  lexer._TOKENSTYLES[token_name], lexer._numstyles = num_styles, num_styles + 1
+  lexer._EXTRASTYLES[token_name] = style
+  -- If the lexer is a proxy or a child that embedded itself, copy this style to
+  -- the parent lexer.
+  if lexer._lexer then lexer._lexer:add_style(token_name, style) end
+end
+
+---
+-- Adds to lexer *lexer* a fold point whose beginning and end tokens are string
+-- *token_name* tokens with string content *start_symbol* and *end_symbol*,
+-- respectively.
+-- In the event that *start_symbol* may or may not be a fold point depending on
+-- context, and that additional processing is required, *end_symbol* may be a
+-- function that ultimately returns `1` (indicating a beginning fold point),
+-- `-1` (indicating an ending fold point), or `0` (indicating no fold point).
+-- That function is passed the following arguments:
+--
+--   * `text`: The text being processed for fold points.
+--   * `pos`: The position in *text* of the beginning of the line currently
+--     being processed.
+--   * `line`: The text of the line currently being processed.
+--   * `s`: The position of *start_symbol* in *line*.
+--   * `symbol`: *start_symbol* itself.
+-- @param lexer The lexer to add a fold point to.
+-- @param token_name The token name of text that indicates a fold point.
+-- @param start_symbol The text that indicates the beginning of a fold point.
+-- @param end_symbol Either the text that indicates the end of a fold point, or
+--   a function that returns whether or not *start_symbol* is a beginning fold
+--   point (1), an ending fold point (-1), or not a fold point at all (0).
+-- @usage lex:add_fold_point(lexer.OPERATOR, '{', '}')
+-- @usage lex:add_fold_point(lexer.KEYWORD, 'if', 'end')
+-- @usage lex:add_fold_point(lexer.COMMENT, '#', lexer.fold_line_comments('#'))
+-- @usage lex:add_fold_point('custom', function(text, pos, line, s, symbol)
+--   ... end)
+-- @name add_fold_point
+function M.add_fold_point(lexer, token_name, start_symbol, end_symbol)
+  if not lexer._FOLDPOINTS then lexer._FOLDPOINTS = {_SYMBOLS = {}} end
+  local symbols = lexer._FOLDPOINTS._SYMBOLS
+  if not symbols[start_symbol] then
+    symbols[#symbols + 1], symbols[start_symbol] = start_symbol, true
+  end
+  if not lexer._FOLDPOINTS[token_name] then
+    lexer._FOLDPOINTS[token_name] = {}
+  end
+  if type(end_symbol) == 'string' then
+    if not symbols[end_symbol] then
+      symbols[#symbols + 1], symbols[end_symbol] = end_symbol, true
+    end
+    lexer._FOLDPOINTS[token_name][start_symbol] = 1
+    lexer._FOLDPOINTS[token_name][end_symbol] = -1
+  else
+    lexer._FOLDPOINTS[token_name][start_symbol] = end_symbol -- function or int
+  end
+  -- If the lexer is a proxy or a child that embedded itself, copy this fold
+  -- point to the parent lexer.
+  if lexer._lexer then
+    lexer._lexer:add_fold_point(token_name, start_symbol, end_symbol)
+  end
+end
+
+-- (Re)constructs `lexer._TOKENRULE`.
+local function join_tokens(lexer)
+  local patterns, order = lexer._RULES, lexer._RULEORDER
+  local token_rule = patterns[order[1]]
+  for i = 2, #order do token_rule = token_rule + patterns[order[i]] end
+  lexer._TOKENRULE = token_rule + M.token(M.DEFAULT, M.any)
+  return lexer._TOKENRULE
+end
+
+-- Metatable for lexer grammars.
+-- These grammars are just tables ultimately passed to `lpeg.P()`.
+local grammar_mt = {__index = {
+  -- Adds lexer *lexer* and any of its embedded lexers to this grammar.
+  -- @param lexer The lexer to add.
+  add_lexer = function(self, lexer)
+    local token_rule = lexer:join_tokens()
+    for i = 1, #lexer._CHILDREN do
+      local child = lexer._CHILDREN[i]
+      if child._CHILDREN then self:add_lexer(child) end
+      local rules = child._EMBEDDEDRULES[lexer._NAME]
+      local rules_token_rule = self['__'..child._NAME] or rules.token_rule
+      self[child._NAME] = (-rules.end_rule * rules_token_rule)^0 *
+                          rules.end_rule^-1 * lpeg_V(lexer._NAME)
+      local embedded_child = '_'..child._NAME
+      self[embedded_child] = rules.start_rule *
+                             (-rules.end_rule * rules_token_rule)^0 *
+                             rules.end_rule^-1
+      token_rule = lpeg_V(embedded_child) + token_rule
+    end
+    self['__'..lexer._NAME] = token_rule -- can contain embedded lexer rules
+    self[lexer._NAME] = token_rule^0
+  end
+}}
+
+-- (Re)constructs `lexer._GRAMMAR`.
+-- @param initial_rule The name of the rule to start lexing with. The default
+--   value is `lexer._NAME`. Multilang lexers use this to start with a child
+--   rule if necessary.
+local function build_grammar(lexer, initial_rule)
+  if not lexer._RULES then return end
+  if lexer._CHILDREN then
+    if not initial_rule then initial_rule = lexer._NAME end
+    local grammar = setmetatable({initial_rule}, grammar_mt)
+    grammar:add_lexer(lexer)
+    lexer._INITIALRULE = initial_rule
+    lexer._GRAMMAR = lpeg_Ct(lpeg_P(grammar))
+  else
+    lexer._GRAMMAR = lpeg_Ct(lexer:join_tokens()^0)
+  end
+end
+
+---
+-- Embeds child lexer *child* in parent lexer *lexer* using patterns
+-- *start_rule* and *end_rule*, which signal the beginning and end of the
+-- embedded lexer, respectively.
+-- @param lexer The parent lexer.
+-- @param child The child lexer.
+-- @param start_rule The pattern that signals the beginning of the embedded
+--   lexer.
+-- @param end_rule The pattern that signals the end of the embedded lexer.
+-- @usage html:embed(css, css_start_rule, css_end_rule)
+-- @usage html:embed(lex, php_start_rule, php_end_rule) -- from php lexer
+-- @name embed
+function M.embed(lexer, child, start_rule, end_rule)
+  if lexer._lexer then lexer = lexer._lexer end -- proxy; get true parent
+  -- Add child rules.
+  if not child._EMBEDDEDRULES then child._EMBEDDEDRULES = {} end
+  if not child._RULES then error('Cannot embed lexer with no rules') end
+  child._EMBEDDEDRULES[lexer._NAME] = {
+    ['start_rule'] = start_rule,
+    token_rule = child:join_tokens(),
+    ['end_rule'] = end_rule
+  }
+  if not lexer._CHILDREN then lexer._CHILDREN = {} end
+  local children = lexer._CHILDREN
+  children[#children + 1] = child
+  -- Add child styles.
+  for token, style in pairs(child._EXTRASTYLES) do
+    lexer:add_style(token, style)
+  end
+  -- Add child fold symbols.
+  if child._FOLDPOINTS then
+    for token_name, symbols in pairs(child._FOLDPOINTS) do
+      if token_name ~= '_SYMBOLS' then
+        for symbol, v in pairs(symbols) do
+          lexer:add_fold_point(token_name, symbol, v)
+        end
+      end
+    end
+  end
+  lexer:build_grammar()
+  child._lexer = lexer -- use parent's tokens if child is embedding itself
+end
+
+---
+-- Lexes a chunk of text *text* (that has an initial style number of
+-- *init_style*) using lexer *lexer*, returning a table of token names and
+-- positions.
+-- @param lexer The lexer to lex text with.
+-- @param text The text in the buffer to lex.
+-- @param init_style The current style. Multiple-language lexers use this to
+--   determine which language to start lexing in.
+-- @return table of token names and positions.
+-- @name lex
+function M.lex(lexer, text, init_style)
+  if not lexer._GRAMMAR then return {M.DEFAULT, #text + 1} end
+  if not lexer._LEXBYLINE then
+    -- For multilang lexers, build a new grammar whose initial_rule is the
+    -- current language.
+    if lexer._CHILDREN then
+      for style, style_num in pairs(lexer._TOKENSTYLES) do
+        if style_num == init_style then
+          local lexer_name = style:match('^(.+)_whitespace') or lexer._NAME
+          if lexer._INITIALRULE ~= lexer_name then
+            lexer:build_grammar(lexer_name)
+          end
+          break
+        end
+      end
+    end
+    return lpeg_match(lexer._GRAMMAR, text)
+  else
+    local tokens = {}
+    local function append(tokens, line_tokens, offset)
+      for i = 1, #line_tokens, 2 do
+        tokens[#tokens + 1] = line_tokens[i]
+        tokens[#tokens + 1] = line_tokens[i + 1] + offset
+      end
+    end
+    local offset = 0
+    local grammar = lexer._GRAMMAR
+    for line in text:gmatch('[^\r\n]*\r?\n?') do
+      local line_tokens = lpeg_match(grammar, line)
+      if line_tokens then append(tokens, line_tokens, offset) end
+      offset = offset + #line
+      -- Use the default style to the end of the line if none was specified.
+      if tokens[#tokens] ~= offset then
+        tokens[#tokens + 1], tokens[#tokens + 2] = 'default', offset + 1
+      end
+    end
+    return tokens
+  end
+end
+
+---
+-- Determines fold points in a chunk of text *text* using lexer *lexer*,
+-- returning a table of fold levels associated with line numbers.
+-- *text* starts at position *start_pos* on line number *start_line* with a
+-- beginning fold level of *start_level* in the buffer.
+-- @param lexer The lexer to fold text with.
+-- @param text The text in the buffer to fold.
+-- @param start_pos The position in the buffer *text* starts at, starting at
+--   zero.
+-- @param start_line The line number *text* starts on.
+-- @param start_level The fold level *text* starts on.
+-- @return table of fold levels associated with line numbers.
+-- @name fold
+function M.fold(lexer, text, start_pos, start_line, start_level)
+  local folds = {}
+  if text == '' then return folds end
+  local fold = M.property_int['fold'] > 0
+  local FOLD_BASE = M.FOLD_BASE
+  local FOLD_HEADER, FOLD_BLANK  = M.FOLD_HEADER, M.FOLD_BLANK
+  if fold and lexer._FOLDPOINTS then
+    local lines = {}
+    for p, l in (text..'\n'):gmatch('()(.-)\r?\n') do
+      lines[#lines + 1] = {p, l}
+    end
+    local fold_zero_sum_lines = M.property_int['fold.on.zero.sum.lines'] > 0
+    local fold_compact = M.property_int['fold.compact'] > 0
+    local fold_points = lexer._FOLDPOINTS
+    local fold_point_symbols = fold_points._SYMBOLS
+    local style_at, fold_level = M.style_at, M.fold_level
+    local line_num, prev_level = start_line, start_level
+    local current_level = prev_level
+    for i = 1, #lines do
+      local pos, line = lines[i][1], lines[i][2]
+      if line ~= '' then
+        if lexer._CASEINSENSITIVEFOLDPOINTS then line = line:lower() end
+        local level_decreased = false
+        for j = 1, #fold_point_symbols do
+          local symbol = fold_point_symbols[j]
+          local word = not symbol:find('[^%w_]')
+          local s, e = line:find(symbol, 1, true)
+          while s and e do
+            --if not word or line:find('^%f[%w_]'..symbol..'%f[^%w_]', s) then
+            if not word or not ((s > 1 and line:find('^[%w_]', s - 1)) or
+                                line:find('^[%w_]', e + 1)) then
+              local symbols = fold_points[style_at[start_pos + pos + s - 1]]
+              local level = symbols and symbols[symbol]
+              if type(level) == 'function' then
+                level = level(text, pos, line, s, symbol)
+              end
+              if type(level) == 'number' then
+                current_level = current_level + level
+                if level < 0 and current_level < prev_level then
+                  -- Potential zero-sum line. If the level were to go back up on
+                  -- the same line, the line may be marked as a fold header.
+                  level_decreased = true
+                end
+              end
+            end
+            s = line:find(symbol, s + 1, true)
+          end
+        end
+        folds[line_num] = prev_level
+        if current_level > prev_level then
+          folds[line_num] = prev_level + FOLD_HEADER
+        elseif level_decreased and current_level == prev_level and
+               fold_zero_sum_lines then
+          if line_num > start_line then
+            folds[line_num] = prev_level - 1 + FOLD_HEADER
+          else
+            -- Typing within a zero-sum line.
+            local level = fold_level[line_num - 1] - 1
+            if level > FOLD_HEADER then level = level - FOLD_HEADER end
+            if level > FOLD_BLANK then level = level - FOLD_BLANK end
+            folds[line_num] = level + FOLD_HEADER
+            current_level = current_level + 1
+          end
+        end
+        if current_level < FOLD_BASE then current_level = FOLD_BASE end
+        prev_level = current_level
+      else
+        folds[line_num] = prev_level + (fold_compact and FOLD_BLANK or 0)
+      end
+      line_num = line_num + 1
+    end
+  elseif fold and (lexer._FOLDBYINDENTATION or
+                   M.property_int['fold.by.indentation'] > 0) then
+    -- Indentation based folding.
+    -- Calculate indentation per line.
+    local indentation = {}
+    for indent, line in (text..'\n'):gmatch('([\t ]*)([^\r\n]*)\r?\n') do
+      indentation[#indentation + 1] = line ~= '' and #indent
+    end
+    -- Find the first non-blank line before start_line. If the current line is
+    -- indented, make that previous line a header and update the levels of any
+    -- blank lines inbetween. If the current line is blank, match the level of
+    -- the previous non-blank line.
+    local current_level = start_level
+    for i = start_line - 1, 0, -1 do
+      local level = M.fold_level[i]
+      if level >= FOLD_HEADER then level = level - FOLD_HEADER end
+      if level < FOLD_BLANK then
+        local indent = M.indent_amount[i]
+        if indentation[1] and indentation[1] > indent then
+          folds[i] = FOLD_BASE + indent + FOLD_HEADER
+          for j = i + 1, start_line - 1 do
+            folds[j] = start_level + FOLD_BLANK
+          end
+        elseif not indentation[1] then
+          current_level = FOLD_BASE + indent
+        end
+        break
+      end
+    end
+    -- Iterate over lines, setting fold numbers and fold flags.
+    for i = 1, #indentation do
+      if indentation[i] then
+        current_level = FOLD_BASE + indentation[i]
+        folds[start_line + i - 1] = current_level
+        for j = i + 1, #indentation do
+          if indentation[j] then
+            if FOLD_BASE + indentation[j] > current_level then
+              folds[start_line + i - 1] = current_level + FOLD_HEADER
+              current_level = FOLD_BASE + indentation[j] -- for any blanks below
+            end
+            break
+          end
+        end
+      else
+        folds[start_line + i - 1] = current_level + FOLD_BLANK
+      end
+    end
+  else
+    -- No folding, reset fold levels if necessary.
+    local current_line = start_line
+    for _ in text:gmatch('\r?\n') do
+      folds[current_line] = start_level
+      current_line = current_line + 1
+    end
+  end
+  return folds
+end
+
+---
+-- Creates a returns a new lexer with the given name.
+-- @param name The lexer's name.
+-- @param opts Table of lexer options. Options currently supported:
+--   * `lex_by_line`: Whether or not the lexer only processes whole lines of
+--     text (instead of arbitrary chunks of text) at a time.
+--     Line lexers cannot look ahead to subsequent lines.
+--     The default value is `false`.
+--   * `fold_by_indentation`: Whether or not the lexer does not define any fold
+--     points and that fold points should be calculated based on changes in line
+--     indentation.
+--     The default value is `false`.
+--   * `case_insensitive_fold_points`: Whether or not fold points added via
+--     `lexer.add_fold_point()` ignore case.
+--     The default value is `false`.
+--   * `inherit`: Lexer to inherit from.
+--     The default value is `nil`.
+-- @usage lexer.new('rhtml', {inherit = lexer.load('html')})
+-- @name new
+function M.new(name, opts)
+  local lexer = {
+    _NAME = assert(name, 'lexer name expected'),
+    _LEXBYLINE = opts and opts['lex_by_line'],
+    _FOLDBYINDENTATION = opts and opts['fold_by_indentation'],
+    _CASEINSENSITIVEFOLDPOINTS = opts and opts['case_insensitive_fold_points'],
+    _lexer = opts and opts['inherit']
+  }
+
+  -- Create the initial maps for token names to style numbers and styles.
+  local token_styles = {}
+  for i = 1, #default do token_styles[default[i]] = i - 1 end
+  for i = 1, #predefined do token_styles[predefined[i]] = i + 31 end
+  lexer._TOKENSTYLES, lexer._numstyles = token_styles, #default
+  lexer._EXTRASTYLES = {}
+
+  return setmetatable(lexer, {__index = {
+    add_rule = M.add_rule, modify_rule = M.modify_rule, get_rule = M.get_rule,
+    add_style = M.add_style, add_fold_point = M.add_fold_point,
+    join_tokens = join_tokens, build_grammar = build_grammar, embed = M.embed,
+    lex = M.lex, fold = M.fold
+  }})
+end
+
+-- Legacy support for older lexers.
+-- Processes the `lex._rules`, `lex._tokenstyles`, and `lex._foldsymbols`
+-- tables.
+-- Since legacy lexers may be processed up to twice, ensure their default styles
+-- and rules are not processed more than once.
+local function process_legacy_lexer(lexer)
+  local function warn(msg) --[[io.stderr:write(msg, "\n")]] end
+  if not lexer._LEGACY then
+    lexer._LEGACY = true
+    warn("lexers as tables are deprecated; use 'lexer.new()'")
+    local token_styles = {}
+    for i = 1, #default do token_styles[default[i]] = i - 1 end
+    for i = 1, #predefined do token_styles[predefined[i]] = i + 31 end
+    lexer._TOKENSTYLES, lexer._numstyles = token_styles, #default
+    lexer._EXTRASTYLES = {}
+    setmetatable(lexer, getmetatable(M.new('')))
+    if lexer._rules then
+      warn("lexer '_rules' table is deprecated; use 'add_rule()'")
+      for i = 1, #lexer._rules do
+        lexer:add_rule(lexer._rules[i][1], lexer._rules[i][2])
+      end
+    end
+  end
+  if lexer._tokenstyles then
+    warn("lexer '_tokenstyles' table is deprecated; use 'add_style()'")
+    for token, style in pairs(lexer._tokenstyles) do
+      -- If this legacy lexer is being processed a second time, only add styles
+      -- added since the first processing.
+      if not lexer._TOKENSTYLES[token] then lexer:add_style(token, style) end
+    end
+  end
+  if lexer._foldsymbols then
+    warn("lexer '_foldsymbols' table is deprecated; use 'add_fold_point()'")
+    for token_name, symbols in pairs(lexer._foldsymbols) do
+      if type(symbols) == 'table' and token_name ~= '_patterns' then
+        for symbol, v in pairs(symbols) do
+          lexer:add_fold_point(token_name, symbol, v)
+        end
+      end
+    end
+    if lexer._foldsymbols._case_insensitive then
+      lexer._CASEINSENSITIVEFOLDPOINTS = true
+    end
+  end
+end
+
+local lexers = {} -- cache of loaded lexers
+---
+-- Initializes or loads and returns the lexer of string name *name*.
+-- Scintilla calls this function in order to load a lexer. Parent lexers also
+-- call this function in order to load child lexers and vice-versa. The user
+-- calls this function in order to load a lexer when using this module as a Lua
+-- library.
+-- @param name The name of the lexing language.
+-- @param alt_name The alternate name of the lexing language. This is useful for
+--   embedding the same child lexer with multiple sets of start and end tokens.
+-- @param cache Flag indicating whether or not to load lexers from the cache.
+--   This should only be `true` when initially loading a lexer (e.g. not from
+--   within another lexer for embedding purposes).
+--   The default value is `false`.
+-- @return lexer object
+-- @name load
+function M.load(name, alt_name, cache)
+  if cache and lexers[alt_name or name] then return lexers[alt_name or name] end
+
+  -- When using this module as a stand-alone module, the `property` and
+  -- `property_int` tables do not exist (they are not useful). Create them in
+  -- order prevent errors from occurring.
+  if not M.property then
+    M.property, M.property_int = {}, setmetatable({}, {
+      __index = function(t, k) return tonumber(M.property[k]) or 0 end,
+      __newindex = function() error('read-only property') end
+    })
+  end
+
+  -- Load the language lexer with its rules, styles, etc.
+  -- However, replace the default `WHITESPACE` style name with a unique
+  -- whitespace style name (and then automatically add it afterwards), since
+  -- embedded lexing relies on these unique whitespace style names. Note that
+  -- loading embedded lexers changes `WHITESPACE` again, so when adding it
+  -- later, do not reference the potentially incorrect value.
+  M.WHITESPACE = (alt_name or name)..'_whitespace'
+  local lexer = dofile(assert(package.searchpath(name, M.path)))
+  assert(lexer, string.format("'%s.lua' did not return a lexer", name))
+  if alt_name then lexer._NAME = alt_name end
+  if not getmetatable(lexer) or lexer._LEGACY then
+    -- A legacy lexer may need to be processed a second time in order to pick up
+    -- any `_tokenstyles` or `_foldsymbols` added after `lexer.embed_lexer()`.
+    process_legacy_lexer(lexer)
+    if lexer._lexer and lexer._lexer._LEGACY then
+      process_legacy_lexer(lexer._lexer) -- mainly for `_foldsymbols` edits
+    end
+  end
+  lexer:add_style((alt_name or name)..'_whitespace', M.STYLE_WHITESPACE)
+
+  -- If the lexer is a proxy or a child that embedded itself, set the parent to
+  -- be the main lexer.
+  if lexer._lexer then
+    lexer = lexer._lexer
+    lexer._NAME = alt_name or name
+  end
+
+  if cache then lexers[alt_name or name] = lexer end
+  return lexer
+end
+
+-- The following are utility functions lexers will have access to.
+
+-- Common patterns.
+M.any = lpeg_P(1)
+M.ascii = lpeg_R('\000\127')
+M.extend = lpeg_R('\000\255')
+M.alpha = lpeg_R('AZ', 'az')
+M.digit = lpeg_R('09')
+M.alnum = lpeg_R('AZ', 'az', '09')
+M.lower = lpeg_R('az')
+M.upper = lpeg_R('AZ')
+M.xdigit = lpeg_R('09', 'AF', 'af')
+M.cntrl = lpeg_R('\000\031')
+M.graph = lpeg_R('!~')
+M.print = lpeg_R(' ~')
+M.punct = lpeg_R('!/', ':@', '[\'', '{~')
+M.space = lpeg_S('\t\v\f\n\r ')
+
+M.newline = lpeg_S('\r\n\f')^1
+M.nonnewline = 1 - M.newline
+M.nonnewline_esc = 1 - (M.newline + '\\') + '\\' * M.any
+
+M.dec_num = M.digit^1
+M.hex_num = '0' * lpeg_S('xX') * M.xdigit^1
+M.oct_num = '0' * lpeg_R('07')^1
+M.integer = lpeg_S('+-')^-1 * (M.hex_num + M.oct_num + M.dec_num)
+M.float = lpeg_S('+-')^-1 *
+          ((M.digit^0 * '.' * M.digit^1 + M.digit^1 * '.' * M.digit^0) *
+           (lpeg_S('eE') * lpeg_S('+-')^-1 * M.digit^1)^-1 +
+           (M.digit^1 * lpeg_S('eE') * lpeg_S('+-')^-1 * M.digit^1))
+
+M.word = (M.alpha + '_') * (M.alnum + '_')^0
+
+---
+-- Creates and returns a token pattern with token name *name* and pattern
+-- *patt*.
+-- If *name* is not a predefined token name, its style must be defined via
+-- `lexer.add_style()`.
+-- @param name The name of token. If this name is not a predefined token name,
+--   then a style needs to be assiciated with it via `lexer.add_style()`.
+-- @param patt The LPeg pattern associated with the token.
+-- @return pattern
+-- @usage local ws = token(lexer.WHITESPACE, lexer.space^1)
+-- @usage local annotation = token('annotation', '@' * lexer.word)
+-- @name token
+function M.token(name, patt)
+  return lpeg_Cc(name) * patt * lpeg_Cp()
+end
+
+---
+-- Creates and returns a pattern that matches a range of text bounded by
+-- *chars* characters.
+-- This is a convenience function for matching more complicated delimited ranges
+-- like strings with escape characters and balanced parentheses. *single_line*
+-- indicates whether or not the range must be on a single line, *no_escape*
+-- indicates whether or not to ignore '\' as an escape character, and *balanced*
+-- indicates whether or not to handle balanced ranges like parentheses and
+-- requires *chars* to be composed of two characters.
+-- @param chars The character(s) that bound the matched range.
+-- @param single_line Optional flag indicating whether or not the range must be
+--   on a single line.
+-- @param no_escape Optional flag indicating whether or not the range end
+--   character may be escaped by a '\\' character.
+-- @param balanced Optional flag indicating whether or not to match a balanced
+--   range, like the "%b" Lua pattern. This flag only applies if *chars*
+--   consists of two different characters (e.g. "()").
+-- @return pattern
+-- @usage local dq_str_escapes = lexer.delimited_range('"')
+-- @usage local dq_str_noescapes = lexer.delimited_range('"', false, true)
+-- @usage local unbalanced_parens = lexer.delimited_range('()')
+-- @usage local balanced_parens = lexer.delimited_range('()', false, false,
+--   true)
+-- @see nested_pair
+-- @name delimited_range
+function M.delimited_range(chars, single_line, no_escape, balanced)
+  local s = chars:sub(1, 1)
+  local e = #chars == 2 and chars:sub(2, 2) or s
+  local range
+  local b = balanced and s or ''
+  local n = single_line and '\n' or ''
+  if no_escape then
+    local invalid = lpeg_S(e..n..b)
+    range = M.any - invalid
+  else
+    local invalid = lpeg_S(e..n..b) + '\\'
+    range = M.any - invalid + '\\' * M.any
+  end
+  if balanced and s ~= e then
+    return lpeg_P{s * (range + lpeg_V(1))^0 * e}
+  else
+    return s * range^0 * lpeg_P(e)^-1
+  end
+end
+
+---
+-- Creates and returns a pattern that matches pattern *patt* only at the
+-- beginning of a line.
+-- @param patt The LPeg pattern to match on the beginning of a line.
+-- @return pattern
+-- @usage local preproc = token(lexer.PREPROCESSOR, lexer.starts_line('#') *
+--   lexer.nonnewline^0)
+-- @name starts_line
+function M.starts_line(patt)
+  return lpeg_Cmt(lpeg_C(patt), function(input, index, match, ...)
+    local pos = index - #match
+    if pos == 1 then return index, ... end
+    local char = input:sub(pos - 1, pos - 1)
+    if char == '\n' or char == '\r' or char == '\f' then return index, ... end
+  end)
+end
+
+---
+-- Creates and returns a pattern that verifies that string set *s* contains the
+-- first non-whitespace character behind the current match position.
+-- @param s String character set like one passed to `lpeg.S()`.
+-- @return pattern
+-- @usage local regex = lexer.last_char_includes('+-*!%^&|=,([{') *
+--   lexer.delimited_range('/')
+-- @name last_char_includes
+function M.last_char_includes(s)
+  s = '['..s:gsub('[-%%%[]', '%%%1')..']'
+  return lpeg_P(function(input, index)
+    if index == 1 then return index end
+    local i = index
+    while input:sub(i - 1, i - 1):match('[ \t\r\n\f]') do i = i - 1 end
+    if input:sub(i - 1, i - 1):match(s) then return index end
+  end)
+end
+
+---
+-- Returns a pattern that matches a balanced range of text that starts with
+-- string *start_chars* and ends with string *end_chars*.
+-- With single-character delimiters, this function is identical to
+-- `delimited_range(start_chars..end_chars, false, true, true)`.
+-- @param start_chars The string starting a nested sequence.
+-- @param end_chars The string ending a nested sequence.
+-- @return pattern
+-- @usage local nested_comment = lexer.nested_pair('/*', '*/')
+-- @see delimited_range
+-- @name nested_pair
+function M.nested_pair(start_chars, end_chars)
+  local s, e = start_chars, lpeg_P(end_chars)^-1
+  return lpeg_P{s * (M.any - s - end_chars + lpeg_V(1))^0 * e}
+end
+
+---
+-- Creates and returns a pattern that matches any single word in string *words*.
+-- *case_insensitive* indicates whether or not to ignore case when matching
+-- words.
+-- This is a convenience function for simplifying a set of ordered choice word
+-- patterns.
+-- If *words* is a multi-line string, it may contain Lua line comments (`--`)
+-- that will ultimately be ignored.
+-- @param words A string list of words separated by spaces.
+-- @param case_insensitive Optional boolean flag indicating whether or not the
+--   word match is case-insensitive. The default value is `false`.
+-- @param word_chars Unused legacy parameter.
+-- @return pattern
+-- @usage local keyword = token(lexer.KEYWORD, word_match[[foo bar baz]])
+-- @usage local keyword = token(lexer.KEYWORD, word_match([[foo-bar foo-baz
+--   bar-foo bar-baz baz-foo baz-bar]], true))
+-- @name word_match
+function M.word_match(words, case_insensitive, word_chars)
+  local word_list = {}
+  if type(words) == 'table' then
+    -- Legacy `word_match(word_list, word_chars, case_insensitive)` form.
+    words = table.concat(words, ' ')
+    word_chars, case_insensitive = case_insensitive, word_chars
+  end
+  for word in words:gsub('%-%-[^\n]+', ''):gmatch('%S+') do
+    word_list[case_insensitive and word:lower() or word] = true
+    for char in word:gmatch('[^%w_]') do
+      if not (word_chars or ''):find(char, 1, true) then
+        word_chars = (word_chars or '')..char
+      end
+    end
+  end
+  local chars = M.alnum + '_'
+  if (word_chars or '') ~= '' then chars = chars + lpeg_S(word_chars) end
+  return lpeg_Cmt(chars^1, function(input, index, word)
+    if case_insensitive then word = word:lower() end
+    return word_list[word] and index or nil
+  end)
+end
+
+-- Deprecated legacy function. Use `parent:embed()` instead.
+-- Embeds child lexer *child* in parent lexer *parent* using patterns
+-- *start_rule* and *end_rule*, which signal the beginning and end of the
+-- embedded lexer, respectively.
+-- @param parent The parent lexer.
+-- @param child The child lexer.
+-- @param start_rule The pattern that signals the beginning of the embedded
+--   lexer.
+-- @param end_rule The pattern that signals the end of the embedded lexer.
+-- @usage lexer.embed_lexer(M, css, css_start_rule, css_end_rule)
+-- @usage lexer.embed_lexer(html, M, php_start_rule, php_end_rule)
+-- @usage lexer.embed_lexer(html, ruby, ruby_start_rule, ruby_end_rule)
+-- @see embed
+-- @name embed_lexer
+function M.embed_lexer(parent, child, start_rule, end_rule)
+  if not getmetatable(parent) then process_legacy_lexer(parent) end
+  if not getmetatable(child) then process_legacy_lexer(child) end
+  parent:embed(child, start_rule, end_rule)
+end
+
+-- Determines if the previous line is a comment.
+-- This is used for determining if the current comment line is a fold point.
+-- @param prefix The prefix string defining a comment.
+-- @param text The text passed to a fold function.
+-- @param pos The pos passed to a fold function.
+-- @param line The line passed to a fold function.
+-- @param s The s passed to a fold function.
+local function prev_line_is_comment(prefix, text, pos, line, s)
+  local start = line:find('%S')
+  if start < s and not line:find(prefix, start, true) then return false end
+  local p = pos - 1
+  if text:sub(p, p) == '\n' then
+    p = p - 1
+    if text:sub(p, p) == '\r' then p = p - 1 end
+    if text:sub(p, p) ~= '\n' then
+      while p > 1 and text:sub(p - 1, p - 1) ~= '\n' do p = p - 1 end
+      while text:sub(p, p):find('^[\t ]$') do p = p + 1 end
+      return text:sub(p, p + #prefix - 1) == prefix
+    end
+  end
+  return false
+end
+
+-- Determines if the next line is a comment.
+-- This is used for determining if the current comment line is a fold point.
+-- @param prefix The prefix string defining a comment.
+-- @param text The text passed to a fold function.
+-- @param pos The pos passed to a fold function.
+-- @param line The line passed to a fold function.
+-- @param s The s passed to a fold function.
+local function next_line_is_comment(prefix, text, pos, line, s)
+  local p = text:find('\n', pos + s)
+  if p then
+    p = p + 1
+    while text:sub(p, p):find('^[\t ]$') do p = p + 1 end
+    return text:sub(p, p + #prefix - 1) == prefix
+  end
+  return false
+end
+
+---
+-- Returns a fold function (to be passed to `lexer.add_fold_point()`) that folds
+-- consecutive line comments that start with string *prefix*.
+-- @param prefix The prefix string defining a line comment.
+-- @usage lex:add_fold_point(lexer.COMMENT, '--',
+--   lexer.fold_line_comments('--'))
+-- @usage lex:add_fold_point(lexer.COMMENT, '//',
+--   lexer.fold_line_comments('//'))
+-- @name fold_line_comments
+function M.fold_line_comments(prefix)
+  local property_int = M.property_int
+  return function(text, pos, line, s)
+    if property_int['fold.line.comments'] == 0 then return 0 end
+    if s > 1 and line:match('^%s*()') < s then return 0 end
+    local prev_line_comment = prev_line_is_comment(prefix, text, pos, line, s)
+    local next_line_comment = next_line_is_comment(prefix, text, pos, line, s)
+    if not prev_line_comment and next_line_comment then return 1 end
+    if prev_line_comment and not next_line_comment then return -1 end
+    return 0
+  end
+end
+
+M.property_expanded = setmetatable({}, {
+  -- Returns the string property value associated with string property *key*,
+  -- replacing any "$()" and "%()" expressions with the values of their keys.
+  __index = function(t, key)
+    return M.property[key]:gsub('[$%%]%b()', function(key)
+      return t[key:sub(3, -2)]
+    end)
+  end,
+  __newindex = function() error('read-only property') end
+})
+
+--[[ The functions and fields below were defined in C.
+
+---
+-- Returns the line number of the line that contains position *pos*, which
+-- starts from 1.
+-- @param pos The position to get the line number of.
+-- @return number
+local function line_from_position(pos) end
+]]
+
+return M