commonmark-spec

CommonMark

CommonMark is a rationalized version of Markdown syntax, with a spec and BSD-licensed reference implementations in C and JavaScript.

Try it now!

For more details, see https://commonmark.org.

This repository contains the spec itself, along with tools for running tests against the spec, and for creating HTML and PDF versions of the spec.

The reference implementations live in separate repositories:

• https://github.com/commonmark/cmark (C)
• https://github.com/commonmark/commonmark.js (JavaScript)

There is a list of third-party libraries in a dozen different languages here.

Running tests against the spec

The spec contains over 500 embedded examples which serve as conformance tests. To run the tests using an executable $PROG:

python3 test/spec_tests.py --program $PROG

If you want to extract the raw test data from the spec without actually running the tests, you can do:

python3 test/spec_tests.py --dump-tests

and you'll get all the tests in JSON format.

JavaScript developers may find it more convenient to use the commonmark-spec npm package, which is published from this repository. It exports an array tests of JSON objects with the format

{
  "markdown": "Foo\nBar\n---\n",
  "html": "<h2>Foo\nBar</h2>\n",
  "section": "Setext headings",
  "number": 65
}

The spec

The source of the spec is spec.txt. This is basically a Markdown file, with code examples written in a shorthand form:

```````````````````````````````` example
Markdown source
.
expected HTML output
````````````````````````````````

To build an HTML version of the spec, do make spec.html. To build a PDF version, do make spec.pdf. For both versions, you must have the lua rock lcmark installed: after installing lua and lua rocks, luarocks install lcmark. For the PDF you must also have xelatex installed.

The spec is written from the point of view of the human writer, not the computer reader. It is not an algorithm---an English translation of a computer program---but a declarative description of what counts as a block quote, a code block, and each of the other structural elements that can make up a Markdown document.

Because John Gruber's canonical syntax description leaves many aspects of the syntax undetermined, writing a precise spec requires making a large number of decisions, many of them somewhat arbitrary. In making them, we have appealed to existing conventions and considerations of simplicity, readability, expressive power, and consistency. We have tried to ensure that "normal" documents in the many incompatible existing implementations of Markdown will render, as far as possible, as their authors intended. And we have tried to make the rules for different elements work together harmoniously. In places where different decisions could have been made (for example, the rules governing list indentation), we have explained the rationale for our choices. In a few cases, we have departed slightly from the canonical syntax description, in ways that we think further the goals of Markdown as stated in that description.

For the most part, we have limited ourselves to the basic elements described in Gruber's canonical syntax description, eschewing extensions like footnotes and definition lists. It is important to get the core right before considering such things. However, we have included a visible syntax for line breaks and fenced code blocks.

Differences from original Markdown

There are only a few places where this spec says things that contradict the canonical syntax description:

• It allows all punctuation symbols to be backslash-escaped, not just the symbols with special meanings in Markdown. We found that it was just too hard to remember which symbols could be escaped.

• It introduces an alternative syntax for hard line breaks, a backslash at the end of the line, supplementing the two-spaces-at-the-end-of-line rule. This is motivated by persistent complaints about the “invisible” nature of the two-space rule.

• Link syntax has been made a bit more predictable (in a backwards-compatible way). For example, Markdown.pl allows single quotes around a title in inline links, but not in reference links. This kind of difference is really hard for users to remember, so the spec allows single quotes in both contexts.

• The rule for HTML blocks differs, though in most real cases it shouldn't make a difference. (See the section on HTML Blocks for details.) The spec's proposal makes it easy to include Markdown inside HTML block-level tags, if you want to, but also allows you to exclude this. It also makes parsing much easier, avoiding expensive backtracking.

• It does not collapse adjacent bird-track blocks into a single blockquote:

  > these are two
  
  > blockquotes
  
  > this is a single
  >
  > blockquote with two paragraphs
  

• Rules for content in lists differ in a few respects, though (as with HTML blocks), most lists in existing documents should render as intended. There is some discussion of the choice points and differences in the subsection of List Items entitled Motivation. We think that the spec's proposal does better than any existing implementation in rendering lists the way a human writer or reader would intuitively understand them. (We could give numerous examples of perfectly natural looking lists that nearly every existing implementation flubs up.)

• Changing bullet characters, or changing from bullets to numbers or vice versa, starts a new list. We think that is almost always going to be the writer's intent.

• The number that begins an ordered list item may be followed by either . or ). Changing the delimiter style starts a new list.

• The start number of an ordered list is significant.

• Fenced code blocks are supported, delimited by either backticks (```) or tildes (~~~).

Contributing

There is a forum for discussing CommonMark; you should use it instead of github issues for questions and possibly open-ended discussions. Use the github issue tracker only for simple, clear, actionable issues.

Authors

The spec was written by John MacFarlane, drawing on

• his experience writing and maintaining Markdown implementations in several languages, including the first Markdown parser not based on regular expression substitutions (pandoc) and the first markdown parsers based on PEG grammars (peg-markdown, lunamark)
• a detailed examination of the differences between existing Markdown implementations using BabelMark 2, and
• extensive discussions with David Greenspan, Jeff Atwood, Vicent Marti, Neil Williams, and Benjamin Dumke-von der Ehe.

Since the first announcement, many people have contributed ideas. Kārlis Gaņģis was especially helpful in refining the rules for emphasis, strong emphasis, links, and images.