Lightweight markup language

A lightweight markup language (LML), also termed a simple or humane markup language, is a markup language with simple, unobtrusive syntax. It is designed to be easy to create using any generic text editor, as well as easy to read in its raw form. Lightweight markup languages are used in applications where it may be necessary to read the raw document as well as the final rendered output.

For instance, a person downloading a software library might prefer to read the documentation in a text editor rather than a web browser. Another application for such languages is to provide for data entry in web-based publishing, such as weblogs and wikis, where the input interface is a simple text box. The server software then converts the input into a common document markup language like HTML.

History

Lightweight markup languages were originally used on text-only displays which could not display characters in italics or bold, so informal methods to convey this information had to be developed. This formatting choice was naturally carried forth to plain-text email communications. Console browsers may also resort to similar display conventions.

In 1986 international standard SGML provided facilities to define and parse lightweight markup languages using grammars and tag implication. The 1998 W3C XML is a profile of SGML that omits these facilities. However, no SGML DTD for any of the languages listed below is known.

Types

Lightweight markup languages can be categorized by their tag types. Like HTML (<b>bold</b>), some languages use named elements that share a common format for start and end tags (e.g. BBCode [b]bold[/b]), whereas proper lightweight markup languages are restricted to ASCII-only punctuation marks and other non-letter symbols for tags, but some also mix both styles (e.g. Textile bq. ) or allow embedded HTML (e.g. Markdown), possibly extended with custom elements (e.g. MediaWiki <ref>source</ref>).

Most languages distinguish between markup for lines or blocks and for shorter spans of texts, but some only support inline markup.

Some markup languages are tailored for a specific purpose, such as documenting computer code (e.g. POD, RD) or being converted to a certain output format (usually HTML) and nothing else, others are more general in application. This includes whether they are oriented on textual presentation or on data serialization.

Presentation oriented languages include AsciiDoc, atx, BBCode, Creole, Crossmark, Epytext, Haml, JsonML, MakeDoc, Markdown, Org-mode, POD, reST, RD, Setext, SiSU, SPIP, Xupl, Texy!, Textile, txt2tags, UDO and Wikitext.

Data serialization oriented languages include Curl (homoiconic, but also reads JSON; every object serializes), JSON, OGDL, and YAML.

Comparison of language features

Comparing language features
Language	HTML export tool	HTML import tool	Tables	Link titles	`class` attribute	`id` attribute
AsciiDoc	Yes	Yes	Yes	Yes	Yes	Yes
BBCode	No	No	Yes	No	No	No
Creole	No	No	Yes	No	No	No
GitHub Flavored Markdown	Yes	No	Yes	Yes	No	No
Markdown	Yes	Yes	Yes/No	Yes	Yes/No	Yes/No
Markdown Extra	Yes	Yes	Yes^[1]	Yes	Yes	Yes
MediaWiki	Yes	Yes	Yes	Yes	Yes	Yes
MultiMarkdown	Yes	No	Yes	Yes	No	No
Org-mode	Yes	Yes^[2]	Yes	Yes	Yes	Yes
PmWiki	No	Yes	Yes	Yes	Yes	Yes
POD	Yes	?	No	Yes	?	?
reStructuredText	Yes	Yes^[2]	Yes	Yes	Yes	auto
Textile	Yes	No	Yes	Yes	Yes	Yes
Texy!	Yes	Yes	Yes	Yes	Yes	Yes
txt2tags	Yes	Yes^[3]	Yes^[4]	Yes	?	?

Markdown's own syntax does not support tables, class attributes, or id attributes; however, since Markdown supports inclusion of native HTML code, these features can be implemented using direct HTML. (Note that some extensions may support these features.)

Comparison of implementation features

Comparing implementations, especially output formats
Language	Implementations	X HTML	Con/La TeX	PDF	DocBook	ODF	EPUB	DOC (X)	LMLs	Other	License
AsciiDoc	Python, Ruby, JavaScript	XHTML	LaTeX	PDF	DocBook	ODF	EPUB	No	—	Man page etc.	GNU GPL, MIT
BBCode	Perl, PHP, C#, Python, Ruby	(X)HTML	No	No	No	No	No	No	—	—	Public Domain
Creole	PHP, Python, Ruby, JavaScript ^[5]	Depends on implementation									CC_BY-SA 1.0
GitHub Flavored Markdown	Haskell (Pandoc)	HTML	LaTeX, ConTeXt	PDF	DocBook	ODF	EPUB	DOC	AsciiDoc, reST	OPML	GPL
GitHub Flavored Markdown	Java,^[6] JavaScript,^[7]^[8]^[9] PHP,^[10]^[11] Python,^[12] Ruby^[13]	HTML^[7]^[8]^[9]^[11]^[12]	No	No	No	No	No	No	—	—	Proprietary
Markdown	Perl (originally), C,^[14]^[15] Python,^[16] JavaScript, Haskell,^[2] Ruby,^[17] C#, Java, PHP	HTML	LaTeX, ConTeXt	PDF	DocBook	ODF	EPUB	RTF	MediaWiki, reST	Man page, S5 etc.	BSD-style & GPL (both)
Markdown Extra	PHP (originally), Python, Ruby	XHTML	No	No	No	No	No	No	—	—	BSD-style & GPL (both)
MediaWiki	Perl, PHP, Haskell	XHTML	No	No	No	No	No	No	—	—	GNU GPL
MultiMarkdown	C, Perl	(X)HTML	LaTeX	PDF	No	ODF	No	DOC, RTF	—	OPML	GPL, MIT
Org-mode	Emacs Lisp, Ruby (parser only), Perl, OCaml	XHTML	LaTeX	PDF	DocBook	ODF	EPUB^[18]	DOCX ^[18]	Markdown	TXT, XOXO, iCalendar, Texinfo, man, contrib: groff, s5, deck.js, Confluence Wiki Markup, TaskJuggler, RSS, FreeMind	GPL
PmWiki	PHP	XHTML 1.0 Transitional	No	No	No	No	No	No	—	—	GNU GPL
POD	Perl	(X)HTML, XML	LaTeX	No	DocBook	No	No	RTF	—	Man page, plain text	Artistic License, Perl's license
reStructuredText	Python,^[19]^[20] Haskell, Java,	HTML, XML	LaTeX	PDF	DocBook	ODF	EPUB	DOC	—	man, S5, Devhelp, QT Help, CHM, JSON	Public Domain
Textile	PHP, Javascript, Java, Perl, Python, Ruby, ASP, C#, Haskell	XHTML	No	No	No	No	No	No	—	—	Textile License
Texy!	PHP, C#	(X)HTML	No	No	No	No	No	No	—	—	GNU GPL v2 License
txt2tags	Python,^[21] PHP^[22]	(X)HTML, SGML	LaTeX	PDF	DocBook	ODF	EPUB	DOC	Creole, AsciiDoc, MediaWiki, MoinMoin, PmWiki, DokuWiki, Google Code Wiki	roff, man, MagicPoint, Lout, PageMaker, ASCII Art, TXT	GPL

Comparison of lightweight markup language syntax

Although usually documented as yielding italic and bold text, most lightweight markup processors output semantic HTML elements em and strong instead. Monospaced text may either result in semantic code or presentational tt elements. Few languages make a distinction, e.g. Textile, or allow the user to configure the output easily, e.g. Texy.

LMLs sometimes differ for multi-word markup where some require the markup characters to replace the inter-word spaces (infix). Some languages require a single character as prefix and suffix, other need doubled or even tripled ones or support both with slightly different meaning, e.g. different levels of emphasis.

Comparing text formatting syntax
Language	Bold	Italic	Monospace	Notes
AsciiDoc	`bold text`	`'italic text'`	`+monospace text+`	Can double operators to apply formatting where there is no word boundary (for example `bold text` yields bold text).
	`_italic text_`	`monospace text`
Creole	`bold text`	`//italic text//`	`{{{monospace text}}}`	Triple curly braces are for nowiki which is optionally monospace in Creole (the choice of the implementor).
Markdown^[23]	`bold text`	`italic text`	`monospace text`	Markdown doesn't use bold and italic tags, but rather `em` (typically italic) and `strong` (typically bold) tags.
`__bold text__`	`_italic text_`	`monospace text`
MediaWiki	`'''bold text'''`	`''italic text''`	`<code>monospace text</code>`	MediaWiki mostly resorts to inline HTML
Org-mode	`bold text`	`/italic text/`	`=code=`	`_underlined_`
`~verbatim~`	`bold text`	`/italic text/`	`+strike-through+`
PmWiki	`'''bold text'''`	`''italic text''`	`@@monospace text@@`
reST	`bold text`	`italic text`	``monospace text``
Setext	`bold text`	`~italic text~`		`_underlined_text_`
Textile^[24]	`strong`	`_emphasis_`	`@monospace text@`	Semantic `strong` and `em` HTML tags
`bold text`	`__italic text__`	Presentational `b` and `i` HTML tags	`@monospace text@`
Texy!	`bold text`	`italic text`	`monospace text`	Texy uses semantic tags by default, but can be configured to use presentational tags.
`//italic text//`	`bold text`		`monospace text`
txt2tags	`bold text`	`//italic text//`	``monospace text``	`__underlined__` `--strike-through--`
POD	`B<bold text>`	`I<italic text>`	`C<monospace text>`	Indented text is also shown as monospaced code.
BBCode	`[b]bold text[/b]`	`[i]italic text[/i]`	`[code]monospace text[/code]`	Formatting works across line breaks.

Heading syntax

Headings are usually available in up to six levels, but the top one is often reserved to contain the same as the document title, which may be set externally. Some documentation may associate levels with divisional types, e.g. part, chapter, section, article or paragraph.

Most LMLs follow one of two styles for headings, either Setext-like underlines or ATX-like line markers, or they support both.

Underlined headings

Level 1 Heading
===============

Level 2 Heading
---------------

Level 3 Heading
~~~~~~~~~~~~~~~

The first style uses underlines, i.e. repeated characters (e.g. equals =, hyphen - or tilde ~, usually at least two or four times) in the line below the heading text.

Underlined heading levels
Chars:	`=`	`-`	`~`	`*`	`#`	`+`	`^`	`_`	`:`	`”`	`’`	`	`<`	`>`	min
Markdown	1	2	No	No	No	No	No	No	No	No	No	No	No	No	?
Setext	1	2	No	No	No	No	No	No	No	No	No	No	No	No	?
AsciiDoc	1	2	3	No	No	No	No	No	No	No	No	No	No	No	?
Texy!	Yes	Yes	No	Yes	Yes	No	No	No	No	No	No	No	No	No	?
reStructuredText	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	?

RST and Texy determine heading levels dynamically, which makes authoring more individual on the one hand, but complicates merges from external sources on the other hand.

Prefixed headings

# Level 1 Heading
## Level 2 Heading ##
### Level 3 Heading ###

The second style is based on repeated markers (e.g. hash #, equals = or asterisk *) at the start of the heading itself, where the number of repetitions indicates the (sometimes inverse) heading level. Most languages also support the reduplication of the markers at the end of the line, but whereas some make them mandatory, others do not even expect their numbers to match.

Line prefix (and suffix) headings
Character:	`=`	`#`	`*`	`!`	`+`	Suffix	Levels	Indentation
AsciiDoc	Yes	No	No	No	No	Optional	1–6	No
Creole	Yes	No	No	No	No	Optional	1–6	No
MediaWiki	Yes	No	No	No	No	Yes	1–6	No
txt2tags	Yes	No	No	No	Yes	Yes	1–6	No
Markdown	No	Yes	No	No	No	Optional	1–6	No
Texy!	No	Yes	No	No	No	Optional	6–1 or 1–6, dynamic	No
Org-mode	No	No	Yes	No	No	No	1– +∞	alternative ^[25]^[26]
PmWiki	No	No	No	Yes	No	Optional	1–6	No

POD and Textile choose the HTML convention of numbered heading levels instead. Org-mode supports indentation as a means of indicating the level. BBCode does not support section headings at all.

Other heading formats
Language	Format
POD	=head1 Level 1 Heading =head2 Level 2 Heading
Textile^[24]	h1. Level 1 Heading h2. Level 2 Heading h3. Level 3 Heading h4. Level 4 Heading h5. Level 5 Heading h6. Level 6 Heading

Link syntax

Hyperlinks can either be added inline, which may clutter the code because of long URLs, or with named alias or numbered id references to lines containing nothing but the address and related attributes and often may be located anywhere in the document. Most languages allow the author to specify text Text to be displayed instead of the plain address http://example.com and some also provide methods to set a different link title Title which may contain more information about the destination.

LMLs that are tailored for special setups, e.g. wikis or code documentation, may automatically generate named anchors (for headings, functions etc.) inside the document, link to related pages (possibly in a different namespace) or provide a textual search for linked keywords.

Most languages employ (double) square or angular brackets to surround links, but hardly any two languages are completely compatible. Many can automatically recognize and parse absolute URLs inside the text without further markup.

Inline hyperlink syntax
Languages	Basic syntax	Text syntax	Title syntax
BBCode, Creole, MediaWiki, PmWiki	`http://example.com`
Textile		`"Text":http://example.com`	`"Text (Title)":http://example.com`
Texy!		`"Text":http://example.com`	`"Text .(Title)":http://example.com`
AsciiDoc		`http://example.com[Text]`
txt2tags	`[http://example.com]`	`[Text http://example.com]`
MediaWiki	`[http://example.com]`	`[http://example.com Text]`
Creole, MediaWiki, PmWiki	`[[Name]]`	`[[Name\|Text]]`
Org-mode	`[[Name]]`	`[[Name][Text]]`
Creole	`[[Namespace:Name]]`	`[[Namespace:Name\|Text]]`
Org-mode	`[[Namespace:Name]]`	`[[Namespace:Name][Text]]`
Creole, PmWiki	`[[http://example.com]]`	`[[http://example.com\|Text]]`
BBCode	`[[http://example.com]]`	`[url]http://example.com[/url]`	`[url=http://example.com]Text[/url]`
Markdown	`<http://example.com>`	`[Text](http://example.com)`	`[Text](http://example.com "Title")`
reStructuredText	`<http://example.com>`	`Text <http://example.com/>`_
POD	`L</Name>`
POD	`L<http://example.com/>`

Reference syntax
Languages	Text syntax	Title syntax
reStructuredText	... Name_ ... .. _Name: http://example.com
Markdown	... [Text][id] ... [id]: http://example.com	... [Text][id] ... [id]: http://example.com "Title"
Textile	... "Text":alias ... [alias]http://example.com	... "Text":alias ... [alias (Title)]http://example.com
Texy!	... "Text":alias ... [alias]: http://example.com	... "Text":alias ... [alias]: http://example.com .(Title)

List syntax

HTML requires an explicit element for the list, specifying its type, and one for each list item, but most lightweight markup languages need only different line prefixes for the bullet points or enumerated items. Some languages rely on indentation for nested lists, others use repeated parent list markers.

Unordered, bullet list items
Characters:	`*`	`-`	`+`	`#`	`.`	`·`	`•`	`_`	`:`	`–`	`—`	indent	skip	nest
Markdown	Yes	Yes	Yes	No	No	No	No	No	No	No	No	0–3	1–3	indent
MediaWiki	Yes	No	No	No	No	No	No	No	No	No	No	0	1+	repeat

Languages differ on whether they support optional or mandatory digits in numbered list items, which kinds of enumerators they understand (e.g. decimal digit 1, roman numerals i or I, alphabetic letters a or A) and whether they support to keep explicit values in the output format. Some Markdown dialects, for instance, will honor a start value other than 1, but ignore any other explicit value.

Ordered, enumerated list items
Chars:	`#1`	`1.`	`1)`	`1]`	`1}`	`(1)`	`[1]`	`{1}`	indent	skip	nest
Markdown	No	1	1	No	No	No	No	No	0–3	1–3	indent
MediaWiki	#	No	No	No	No	No	No	No	0	1+	repeat

References

↑ "PHP Markdown Extra". Michelf.com. Retrieved 2013-10-08.
1 2 3 Pandoc, which is written in Haskell, parses Markdown (in two forms) and ReStructuredText, as well as HTML and LaTeX; it writes from any of these formats to HTML, RTF, LaTeX, ConTeXt, OpenDocument, EPUB and several other formats, including (via LaTeX) PDF.
↑ "Html2wiki txt2tags module". cpan.org. Retrieved 2014-01-30.
↑ "Txt2tags User Guide". Txt2tags.sourceforge.net. Retrieved 2013-10-08.
↑ "Converters". WikiCreole. Retrieved 2013-10-08.
↑ pegdown : A Java library for Markdown processing
1 2 gfms : Github Flavored Markdown Server
1 2 marked : A full-featured markdown parser and compiler, written in JavaScript. Built for speed.
1 2 node-gfm : GitHub flavored markdown to html converter
↑ Parsedown : Markdown parser written in PHP
1 2 Ciconia : Markdown parser written in PHP
1 2 Grip : GitHub Readme Instant Preview
↑ github-markdown : Self-contained Markdown parser for GitHub
↑ peg-markdown is an implementation of markdown in C.
↑ Discount is also an implementation of markdown in C.
↑ "Python-Markdown". Github.com. Retrieved 2013-10-08.
↑ Bruce Williams <http://codefluency.com>, for Ruby Central <http://rubycentral.org>. "kramdown: Project Info". RubyForge. Retrieved 2013-10-08.
1 2 "Via ox-pandoc and pandoc itself".
↑ Docutils is an implementation of ReStructuredText in Python
↑ Sphinx is an implementation of ReStructuredText in Python and Docutils with a number of output format Builders
↑ Aurelio Jargas www.aurelio.net (2012-01-11). "txt2tags". txt2tags. Retrieved 2013-10-08.
↑ "txt2tags.class.php - online convertor [sic]". Txt2tags.org. Retrieved 2013-10-08.
↑ "Markdown Syntax". Daringfireball.net. Retrieved 2013-10-08.
1 2 Textile Syntax
↑ "using org-adapt-indentation".
↑ "using org-indent-mode or org-indent".

External links

Wikibooks has a book on the topic of: Curl

Markup language varieties

Document General-purpose Geographic information Lightweight Mathematical Page description Playlist User interface Vector graphics XML Unclassified

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