Scintillua API Documentation

The lexer Lua Module


Lexes Scintilla documents and source code with Lua and LPeg.

Writing Lua Lexers

Lexers recognize and tag elements of source code for syntax highlighting. Scintilla (the editing component behind Textadept and SciTE) 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.

While lexers can be written in plain Lua, Scintillua prefers using Parsing Expression Grammars, or PEGs, composed with the Lua LPeg library. As a result, this document is devoted to writing LPeg lexers. 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 number of 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 also 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 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 and SciTE editors. Finally there are comments on lexer performance and limitations.

Lexer Basics

The lexers/ directory contains all of Scintillua’s Lua lexers, including any new ones you write. 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, Scintillua uses “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 ‘?’ with the name of your lexer. Consider this snippet from the template:

-- ? LPeg lexer.

local lexer = lexer
local P, S = lpeg.P, lpeg.S

local lex = lexer.new(...)

[... lexer rules ...]

-- Identifier.
local identifier = lex:tag(lexer.IDENTIFIER, lexer.word)
lex:add_rule('identifier', identifier)

[... more lexer rules ...]

return lex

The first line of code is a Lua convention to store a global variable into a local variable for quick access. The second line simply defines often used convenience variables. The third and last lines define and return the lexer object Scintilla uses; they are very important and must be part of every lexer. Note the ... passed to lexer.new() is literal: the lexer will assume the name of its filename or an alternative name specified by lexer.load() in embedded lexer applications. The fourth line uses something called a “tag”, an essential component of lexers. You will learn about tags shortly. The fifth line defines a lexer grammar rule, which you will learn about later. (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.

Tags

Take a moment to think about your programming language’s structure. What kind of key elements does it have? Most languages have elements like keywords, strings, and comments. The lexer’s job is to break down source code into these elements and “tag” them for syntax highlighting. Therefore, tags are an essential component of lexers. It is up to you how specific your lexer is when it comes to tagging elements. 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, tags the following elements: keywords, functions, constants, identifiers, strings, comments, numbers, labels, attributes, and operators. Even though functions and constants 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, LPeg patterns that match particular sequences of characters are tagged with a tag name using the the lexer.tag() function. Let us examine the “identifier” tag used in the template shown earlier:

local identifier = lex:tag(lexer.IDENTIFIER, lexer.word)

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:

lex:tag('identifier', (lpeg.R('AZ', 'az')  + '_') * (lpeg.R('AZ', 'az', '09') + '_')^0)

The lexer module actually provides a convenient list of common tag names and common LPeg patterns for you to use. Tag names for programming languages include (but are not limited to) lexer.DEFAULT, lexer.COMMENT, lexer.STRING, lexer.NUMBER, lexer.KEYWORD, lexer.IDENTIFIER, lexer.OPERATOR, lexer.ERROR, lexer.PREPROCESSOR, lexer.CONSTANT, lexer.CONSTANT_BUILTIN, lexer.VARIABLE, lexer.VARIABLE_BUILTIN, lexer.FUNCTION, lexer.FUNCTION_BUILTIN, lexer.FUNCTION_METHOD, lexer.CLASS, lexer.TYPE, lexer.LABEL, lexer.REGEX, lexer.EMBEDDED, and lexer.ANNOTATION. Tag names for markup languages include (but are not limited to) lexer.TAG, lexer.ATTRIBUTE, lexer.HEADING, lexer.BOLD, lexer.ITALIC, lexer.UNDERLINE, lexer.CODE, lexer.LINK, lexer.REFERENCE, and lexer.LIST. Patterns include lexer.any, lexer.alpha, lexer.digit, lexer.alnum, lexer.lower, lexer.upper, lexer.xdigit, lexer.graph, lexer.print, lexer.punct, lexer.space, lexer.newline, lexer.nonnewline, lexer.dec_num, lexer.hex_num, lexer.oct_num, lexer.bin_num, lexer.integer, lexer.float, lexer.number, and lexer.word. You may use your own tag names if none of the above fit your language, but an advantage to using predefined tag names is that the language elements your lexer recognizes will inherit any universal syntax highlighting color theme that your editor uses. You can also “subclass” existing tag names by appending a ‘.subclass’ string to them. For example, the HTML lexer tags unknown tags as lexer.TAG .. '.unknown'. Editors have the ability to style those subclassed tags in a different way than normal tags, or fall back to styling them as normal tags.

Example Tags

So, how might you recognize and tag elements like keywords, comments, and strings? Here are some examples.

Keywords

Instead of matching n keywords with n P('keyword_n') ordered choices, use one of of the following methods:

  1. Use the convenience function lexer.word_match() optionally coupled with lexer.set_word_list(). It is much easier and more efficient to write word matches like:

    local keyword = lex:tag(lexer.KEYWORD, lex:word_match(lexer.KEYWORD)) […] lex:set_word_list(lexer.KEYWORD, { ‘keyword_1’, ‘keyword_2’, …, ‘keyword_n’ })

    local case_insensitive_word = lex:tag(lexer.KEYWORD, lex:word_match(lexer.KEYWORD, true)) […] lex:set_word_list(lexer.KEYWORD, { ‘KEYWORD_1’, ‘keyword_2’, …, ‘KEYword_n’ })

    local hyphenated_keyword = lex:tag(lexer.KEYWORD, lex:word_match(lexer.KEYWORD)) […] lex:set_word_list(lexer.KEYWORD, { ‘keyword-1’, ‘keyword-2’, …, ‘keyword-n’ })

The benefit of using this method is that other lexers that inherit from, embed, or embed themselves into your lexer can set, replace, or extend these word lists. For example, the TypeScript lexer inherits from JavaScript, but extends JavaScript’s keyword and type lists with more options.

This method also allows applications that use your lexer to extend or replace your word lists. For example, the Lua lexer includes keywords and functions for the latest version of Lua (5.4 at the time of writing). However, editors using that lexer might want to use keywords from Lua version 5.1, which is still quite popular.

Note that calling lex:set_word_list() is completely optional. Your lexer is allowed to expect the editor using it to supply word lists. Scintilla-based editors can do so via Scintilla’s ILexer5 interface.

  1. Use the lexer-agnostic form of lexer.word_match():

    local keyword = lex:tag(lexer.KEYWORD, lexer.word_match{
      'keyword_1', 'keyword_2', ..., 'keyword_n'
    })
    
    local case_insensitive_keyword = lex:tag(lexer.KEYWORD, lexer.word_match({
      'KEYWORD_1', 'keyword_2', ..., 'KEYword_n'
    }, true))
    
    local hyphened_keyword = lex:tag(lexer.KEYWORD, lexer.word_match{
      'keyword-1', 'keyword-2', ..., 'keyword-n'
    })
    

    For short keyword lists, you can use a single string of words. For example:

    local keyword = lex:tag(lexer.KEYWORD, lexer.word_match('key_1 key_2 ... key_n'))
    

    You can use this method for static word lists that do not change, or where it does not make sense to allow applications or other lexers to extend or replace a word list.

Comments

Line-style comments with a prefix character(s) are easy to express:

local shell_comment = lex:tag(lexer.COMMENT, lexer.to_eol('#'))
local c_line_comment = lex:tag(lexer.COMMENT, lexer.to_eol('//', true))

The comments above start with a ‘#’ or “//” and go to the end of the line (EOL). 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 = lex:tag(lexer.COMMENT, lexer.range('/*', '*/'))

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

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. lexer.range() handles escapes inherently.

local dq_str = lexer.range('"')
local sq_str = lexer.range("'")
local string = lex:tag(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 predefined LPeg pattern:

local number = lex:tag(lexer.NUMBER, lexer.number)

However, some languages allow postfix characters on integers.

local integer = P('-')^-1 * (lexer.dec_num * S('lL')^-1)
local number = lex:tag(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 syntactic structure. For example, comments usually cannot appear within a string, and valid identifiers (like variable names) cannot be keywords. In Lua lexers, grammars consist of LPeg pattern rules, many of which are tagged. Recall from the lexer template the lexer.add_rule() call, which adds a rule to the lexer’s grammar:

lex:add_rule('identifier', identifier)

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 identifier tokens under a rule named “identifier”.

To illustrate the importance of rule order, here is an example of a simplified Lua lexer:

lex:add_rule('keyword', lex:tag(lexer.KEYWORD, ...))
lex:add_rule('identifier', lex:tag(lexer.IDENTIFIER, ...))
lex:add_rule('string', lex:tag(lexer.STRING, ...))
lex:add_rule('comment', lex:tag(lexer.COMMENT, ...))
lex:add_rule('number', lex:tag(lexer.NUMBER, ...))
lex:add_rule('label', lex:tag(lexer.LABEL, ...))
lex:add_rule('operator', lex:tag(lexer.OPERATOR, ...))

Notice 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 would be incorrectly tagged (and likewise incorrectly highlighted) 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('keyword', keyword)
...
lex:add_rule('error', lex:tag(lexer.ERROR, lexer.any))

This identifies and tags any character not matched by an existing rule as a lexer.ERROR.

Even though the rules defined in the examples above contain a single tagged pattern, rules may consist of multiple tagged patterns. For example, the rule for an HTML tag could consist of a tagged tag followed by an arbitrary number of tagged attributes, separated by whitespace. This allows the lexer to produce all tags separately, but in a single, convenient rule. That rule might look something like this:

local ws = lex:get_rule('whitespace') -- predefined rule for all lexers
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 tagged patterns and grammar rules. These patterns match language elements like keywords, comments, and strings, and rules dictate the order in which patterns are matched. At your disposal are a number of convenience patterns and functions for rapidly creating a lexer. If you choose to use predefined tag names (or perhaps even subclassed names) for your patterns, you do not have to update your editor’s theme to specify how to syntax-highlight those patterns. Your language’s elements will inherit the default syntax highlighting color theme your editor uses.

Advanced Techniques

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 to subsequent lines.

Embedded Lexers

Scintillua 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 = lex:tag('php_tag', '<?php ')
local php_end_rule = lex:tag('php_tag', '?>')
html:embed(lex, php_start_rule, php_end_rule)

Note that the use of a ‘php_tag’ tag will require the editor using the lexer to specify how to highlight text with that tag. In order to avoid this, you could use the lexer.EMBEDDED tag instead, or perhaps lexer.TAG .. '.php' if your application supports it.

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 the Textadept and SciTE editors for example, 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 tagged as an lexer.OPERATOR is a fold point. Likewise, the second assignment states that any “/*” or “*/” that the lexer tagged as part of a lexer.COMMENT is a fold point. The lexer does not consider any occurrences of these characters outside their tagged elements (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 tagged element is a fold point, pass a function to lex:add_fold_point() that returns an integer. A return value of 1 indicates the element is a beginning fold point and a return value of -1 indicates the element is an ending fold point. A return value of 0 indicates the element is not a fold point. For example:

local function fold_strange_element(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_element', '|', fold_strange_element)

Any time the lexer encounters a ‘|’ that is tagged as a “strange_element”, it calls the fold_strange_element 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

Place 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.

SciTE

Create a .properties file for your lexer and import it in either your SciTEUser.properties or SciTEGlobal.properties. The contents of the .properties file should contain:

file.patterns.[lexer_name]=[file_patterns]
lexer.$(file.patterns.[lexer_name])=scintillua.[lexer_name]

where [lexer_name] is the name of your lexer (minus the .lua extension) and [file_patterns] is a set of file extensions to use your lexer for.

SciTE assigns styles to tag names in order to perform syntax highlighting. Since the set of tag names used for a given language changes, your .properties file should specify styles for tag names instead of style numbers. For example:

scintillua.styles.default=fore:#000000,back:#FFFFFF
scintillua.styles.keyword=fore:#00007F,bold
scintillua.styles.string=fore:#7F007F
scintillua.styles.my_tag=

Migrating Legacy Lexers

Legacy lexers are of the form:

local lexer = require('lexer')
local token, word_match = lexer.token, lexer.word_match
local P, S = lpeg.P, lpeg.S

local lex = lexer.new('?')

-- Whitespace.
lex:add_rule('whitespace', token(lexer.WHITESPACE, lexer.space^1))

-- Keywords.
lex:add_rule('keyword', token(lexer.KEYWORD, word_match{
  [...]
}))

[... other rule definitions ...]

-- Custom.
lex:add_rule('custom_rule', token('custom_token', ...))
lex:add_style('custom_token', lexer.styles.keyword .. {bold = true})

-- Fold points.
lex:add_fold_point(lexer.OPERATOR, '{', '}')

return lex

While Scintillua will mostly handle such legacy lexers just fine without any changes, it is recommended that you migrate yours. The migration process is fairly straightforward:

  1. lexer exists in the default lexer environment, so require('lexer') should be replaced by simply lexer. (Keep in mind local lexer = lexer is a Lua idiom.)
  2. Every lexer created using lexer.new() should no longer specify a lexer name by string, but should instead use ... (three dots), which evaluates to the lexer’s filename or alternative name in embedded lexer applications.
  3. Every lexer created using lexer.new() now includes a rule to match whitespace. Unless your lexer has significant whitespace, you can remove your legacy lexer’s whitespace token and rule. Otherwise, your defined whitespace rule will replace the default one.
  4. The concept of tokens has been replaced with tags. Instead of calling a token() function, call lex:tag() instead.
  5. Lexers now support replaceable word lists. Instead of calling lexer.word_match() with large word lists, call it as an instance method with an identifier string (typically something like lexer.KEYWORD). Then at the end of the lexer (before return lex), call lex:set_word_list() with the same identifier and the usual list of words to match. This allows users of your lexer to call lex:set_word_list() with their own set of words should they wish to.
  6. Lexers no longer specify styling information. Remove any calls to lex:add_style().
  7. lexer.last_char_includes() has been deprecated in favor of the new lexer.after_set(). Use the character set and pattern as arguments to that new function.

As an example, consider the following sample legacy lexer:

local lexer = require('lexer')
local token, word_match = lexer.token, lexer.word_match
local P, S = lpeg.P, 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', 'quux'))
lex:add_style('custom', lexer.styles.keyword .. {bold = true})
lex:add_rule('identifier', token(lexer.IDENTIFIER, lexer.word))
lex:add_rule('string', token(lexer.STRING, lexer.range('"')))
lex:add_rule('comment', token(lexer.COMMENT, lexer.to_eol('#')))
lex:add_rule('number', token(lexer.NUMBER, lexer.number))
lex:add_rule('operator', token(lexer.OPERATOR, S('+-*/%^=<>,.()[]{}')))

lex:add_fold_point(lexer.OPERATOR, '{', '}')

return lex

Following the migration steps would yield:

local lexer = lexer
local P, S = lpeg.P, lpeg.S

local lex = lexer.new(...)

lex:add_rule('keyword', lex:tag(lexer.KEYWORD, lex:word_match(lexer.KEYWORD)))
lex:add_rule('custom', lex:tag('custom', 'quux'))
lex:add_rule('identifier', lex:tag(lexer.IDENTIFIER, lexer.word))
lex:add_rule('string', lex:tag(lexer.STRING, lexer.range('"')))
lex:add_rule('comment', lex:tag(lexer.COMMENT, lexer.to_eol('#')))
lex:add_rule('number', lex:tag(lexer.NUMBER, lexer.number))
lex:add_rule('operator', lex:tag(lexer.OPERATOR, S('+-*/%^=<>,.()[]{}')))

lex:add_fold_point(lexer.OPERATOR, '{', '}')

lex:set_word_list(lexer.KEYWORD, {'foo', 'bar', 'baz'})

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. Actually painting the syntax highlighted text to the screen is often more expensive than the lexing operation. 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 themselves into their parent languages because the parent’s tagged patterns do not support start and end rules. This mostly goes unnoticed, but code like

<div id="<?php echo $id; ?>">

will not style correctly. Also, these types of languages cannot currently embed themselves into their parent’s child languages either.

A language cannot embed itself into something like an interpolated string because it is possible that if lexing starts within the embedded entity, it will not be detected as such, so a child to parent transition cannot happen. For example, the following Ruby code will not style correctly:

sum = "1 + 2 = #{1 + 2}"

Also, there is the potential for recursion for languages embedding themselves within themselves.

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 processing and tagging 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 provided inspiration, and thanks to Roberto Ierusalimschy for LPeg.

Fields defined by lexer

lexer.ANNOTATION (string)

The tag name for annotation elements.

lexer.ATTRIBUTE (string)

The tag name for function attribute elements, typically in markup.

lexer.BOLD (string)

The tag name for bold elements, typically in markup.

lexer.CLASS (string)

The tag name for class elements.

lexer.CODE (string)

The tag name for code elements, typically in markup.

lexer.COMMENT (string)

The tag name for comment elements.

lexer.CONSTANT (string)

The tag name for constant elements.

lexer.CONSTANT_BUILTIN (string)

The tag name for builtin constant elements.

lexer.DEFAULT (string)

The tag name for default elements.

lexer.EMBEDDED (string)

The tag name for embedded elements.

lexer.ERROR (string)

The tag name for error elements.

lexer.FOLD_BASE (number)

The initial (root) fold level.

lexer.FOLD_BLANK (number)

Flag indicating that the line is blank.

lexer.FOLD_HEADER (number)

Flag indicating the line is fold point.

lexer.FUNCTION (string)

The tag name for function elements.

lexer.FUNCTION_BUILTIN (string)

The tag name for builtin function elements.

lexer.FUNCTION_METHOD (string)

The tag name for function method elements.

lexer.HEADING (string)

The tag name for heading elements, typically in markup.

lexer.IDENTIFIER (string)

The tag name for identifier elements.

lexer.ITALIC (string)

The tag name for builtin italic elements, typically in markup.

lexer.KEYWORD (string)

The tag name for keyword elements.

lexer.LABEL (string)

The tag name for label elements.

The tag name for link elements, typically in markup.

lexer.LIST (string)

The tag name for list item elements, typically in markup.

lexer.NUMBER (string)

The tag name for number elements.

lexer.OPERATOR (string)

The tag name for operator elements.

lexer.PREPROCESSOR (string)

The tag name for preprocessor elements.

lexer.REFERENCE (string)

The tag name for reference elements, typically in markup.

lexer.REGEX (string)

The tag name for regex elements.

lexer.STRING (string)

The tag name for string elements.

lexer.TAG (string)

The tag name for function tag elements, typically in markup.

lexer.TYPE (string)

The tag name for type elements.

lexer.UNDERLINE (string)

The tag name for underlined elements, typically in markup.

lexer.VARIABLE (string)

The tag name for variable elements.

lexer.VARIABLE_BUILTIN (string)

The tag name for builtin variable elements.

lexer.alnum (pattern)

A pattern that matches any alphanumeric character (‘A’-‘Z’, ‘a’-‘z’, ‘0’-‘9’).

lexer.alpha (pattern)

A pattern that matches any alphabetic character (‘A’-‘Z’, ‘a’-‘z’).

lexer.any (pattern)

A pattern that matches any single character.

lexer.bin_num (pattern)

A pattern that matches a binary number.

lexer.dec_num (pattern)

A pattern that matches a decimal number.

lexer.digit (pattern)

A pattern that matches any digit (‘0’-‘9’).

lexer.float (pattern)

A pattern that matches a floating point number.

lexer.fold_level (table, Read-only)

Table of fold level bit-masks for line numbers starting from 1. Fold level masks are composed of an integer level combined with any of the following bits:

lexer.graph (pattern)

A pattern that matches any graphical character (‘!’ to ‘~’).

lexer.hex_num (pattern)

A pattern that matches a hexadecimal number.

lexer.indent_amount (table, Read-only)

Table of indentation amounts in character columns, for line numbers starting from 1.

lexer.integer (pattern)

A pattern that matches either a decimal, hexadecimal, octal, or binary number.

lexer.line_state (table)

Table of integer line states for line numbers starting from 1. Line states can be used by lexers for keeping track of persistent states. For example, the output lexer uses this to mark lines that have warnings or errors.

lexer.lower (pattern)

A pattern that matches any lower case character (‘a’-‘z’).

lexer.newline (pattern)

A pattern that matches a sequence of end of line characters.

lexer.nonnewline (pattern)

A pattern that matches any single, non-newline character.

lexer.num_user_word_lists (number)

The number of word lists to add as rules to every lexer created by lexer.new(). These word lists are intended to be set by users outside the lexer. Each word in a list is tagged with the name userlistN, where N is the index of the list. The default value is 2.

lexer.number (pattern)

A pattern that matches a typical number, either a floating point, decimal, hexadecimal, octal, or binary number.

lexer.oct_num (pattern)

A pattern that matches an octal number.

lexer.property (table)

Map of key-value string pairs.

lexer.property_int (table, Read-only)

Map of key-value pairs with values interpreted as numbers, or 0 if not found.

lexer.punct (pattern)

A pattern that matches any punctuation character (‘!’ to ‘/’, ‘:’ to ‘@’, ‘[’ to ‘’’, ‘{‘ to ‘~’).

lexer.space (pattern)

A pattern that matches any whitespace character (‘\t’, ‘\v’, ‘\f’, ‘\n’, ‘\r’, space).

lexer.style_at (table, Read-only)

Table of style names at positions in the buffer starting from 1.

lexer.upper (pattern)

A pattern that matches any upper case character (‘A’-‘Z’).

lexer.word (pattern)

A pattern that matches a typical word. Words begin with a letter or underscore and consist of alphanumeric and underscore characters.

lexer.xdigit (pattern)

A pattern that matches any hexadecimal digit (‘0’-‘9’, ‘A’-‘F’, ‘a’-‘f’).

Functions defined by lexer

lexer.add_fold_point(lexer, tag_name, start_symbol, end_symbol)

Adds to lexer lexer a fold point whose beginning and end points are tagged with string tag_name tags and have 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:

Fields:

Usage:

lexer.add_rule(lexer, id, rule)

Adds pattern rule identified by string id to the ordered list of rules for lexer lexer.

Fields:

See also:

lexer.after_set(set, patt, skip)

Creates and returns a pattern that matches pattern patt only when it comes after one of the characters in string set (or when there are no characters behind patt), skipping over any characters in string skip, which is whitespace by default.

Fields:

Usage:

lexer.bin_num_(c)

Returns a pattern that matches a binary number, whose digits may be separated by character c.

Fields:

lexer.dec_num_(c)

Returns a pattern that matches a decimal number, whose digits may be separated by character c.

Fields:

lexer.detect(filename, line)

Returns the name of the lexer often associated with filename filename and/or content line line.

Fields:

Return:

See also:

lexer.embed(lexer, child, start_rule, end_rule)

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.

Fields:

Usage:

lexer.float_(c)

Returns a pattern that matches a floating point number, whose digits may be separated by character c.

Fields:

lexer.fold(lexer, text, start_line, start_level)

Determines fold points in a chunk of text text using lexer lexer, returning a table of fold levels associated with line numbers. text starts on line number start_line with a beginning fold level of start_level in the buffer.

Fields:

Return:

lexer.fold_consecutive_lines(prefix)

Returns for lexer.add_fold_point() the parameters needed to fold consecutive lines that start with string prefix.

Fields:

Usage:

lexer.get_rule(lexer, id)

Returns the rule identified by string id.

Fields:

Return:

lexer.hex_num_(c)

Returns a pattern that matches a hexadecimal number, whose digits may be separated by character c.

Fields:

lexer.integer_(c)

Returns a pattern that matches either a decimal, hexadecimal, octal, or binary number, whose digits may be separated by character c.

Fields:

lexer.lex(lexer, text, init_style)

Lexes a chunk of text text (that has an initial style number of init_style) using lexer lexer, returning a list of tag names and positions.

Fields:

Return:

lexer.line_from_position(pos)

Returns the line number (starting from 1) of the line that contains position pos, which starts from 1.

Fields:

Return:

lexer.load(name, alt_name)

Initializes or loads and then 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 Scintillua as a Lua library.

Fields:

Return:

lexer.modify_rule(lexer, id, rule)

Replaces in lexer lexer the existing rule identified by string id with pattern rule.

Fields:

lexer.new(name, opts)

Creates a returns a new lexer with the given name.

Fields:

Usage:

lexer.number_(c)

Returns a pattern that matches a typical number, either a floating point, decimal, hexadecimal, octal, or binary number, and whose digits may be separated by character c.

Fields:

lexer.oct_num_(c)

Returns a pattern that matches an octal number, whose digits may be separated by character c.

Fields:

lexer.range(s, e, single_line, escapes, balanced)

Creates and returns a pattern that matches a range of text bounded by strings or patterns s and e. This is a convenience function for matching more complicated ranges like strings with escape characters, balanced parentheses, and block comments (nested or not). e is optional and defaults to s. single_line indicates whether or not the range must be on a single line; escapes indicates whether or not to allow ‘' as an escape character; and balanced indicates whether or not to handle balanced ranges like parentheses, and requires s and e to be different.

Fields:

Usage:

Return:

lexer.set_word_list(lexer, name, word_list, append)

Sets in lexer lexer the word list identified by string or number name to string or list word_list, appending to any existing word list if append is true. This only has an effect if lexer uses word_match() to reference the given list. Case-insensitivity is specified by word_match().

Fields:

See also:

lexer.starts_line(patt, allow_indent)

Creates and returns a pattern that matches pattern patt only at the beginning of a line, or after any line indentation if allow_indent is true.

Fields:

Usage:

Return:

lexer.tag(lexer, name, patt)

Creates and returns a pattern that tags pattern patt with name name in lexer lexer. If name is not a predefined tag name, its Scintilla style will likely need to be defined by the editor or theme using this lexer.

Fields:

Usage:

Return:

lexer.to_eol(prefix, escape)

Creates and returns a pattern that matches from string or pattern prefix until the end of the line. escape indicates whether the end of the line can be escaped with a ‘' character.

Fields:

Usage:

Return:

lexer.word_match(lexer, word_list, case_insensitive)

Either returns a pattern for lexer lexer (if given) that matches one word in the word list identified by string word_list, ignoring case if case_sensitive is true, or, if lexer is not given, creates and returns a pattern that matches any single word in list or string word_list, ignoring case if case_insensitive is true. This is a convenience function for simplifying a set of ordered choice word patterns and potentially allowing downstream users to configure word lists. If there is ultimately no word list set via set_word_list(), no error will be raised, but the returned pattern will not match anything.

Fields:

Usage:

Return:

See also:

Tables defined by lexer

lexer.detect_extensions

Map of file extensions, without the ‘.’ prefix, to their associated lexer names. This map has precedence over Scintillua’s built-in map.

See also:

lexer.detect_patterns

Map of line patterns to their associated lexer names. These are Lua string patterns, not LPeg patterns. This map has precedence over Scintillua’s built-in map.

See also: