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CommonMark is a specification of Markdown syntax, together with BSD3-licensed implementations (stmd) in C and JavaScript.

The implementations

The C implementation provides both a library and a standalone program stmd that converts Markdown to HTML. It is written in standard C99 and has no library dependencies. (However, if you check it out from the repository, you'll need re2c to generate scanners.c from This is only a build dependency for developers, since scanners.c can be provided in a released source tarball.)

Usage:   stmd [FILE*]
Options: --help, -h    Print usage information
         --ast         Print AST instead of HTML
         --version     Print version

The JavaScript implementation is a single JavaScript file, with no dependencies, that can be linked to in an HTML page. A standalone version (using node.js) is also provided (js/markdown), and there is a "dingus" for playing with it interactively. (make dingus will start this.)

Try it now!

The spec contains over 400 embedded examples which serve as conformance tests. To run the tests for stmd, do make test. To run them for another Markdown program, say myprog, do make test PROG=myprog. To run the tests for stmd.js, do make testjs.

The spec

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

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. Both these commands require that pandoc is installed, and creating a PDF requires a latex installation.

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, I have appealed to existing conventions and considerations of simplicity, readability, expressive power, and consistency. I 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 I 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), I have explained the rationale for my choices. In a few cases, I have departed slightly from the canonical syntax description, in ways that I think further the goals of Markdown as stated in that description.

For the most part, I have limited myself 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, I have included a visible syntax for line breaks and fenced code blocks.

In all of this, I have been guided by eight years experience writing 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). Maintaining these projects and responding to years of user feedback have given me a good sense of the complexities involved in parsing Markdown, and of the various design decisions that can be made. I have also explored differences between Markdown implementations extensively using babelmark 2. In the early phases of working out the spec, I benefited greatly from collaboration with David Greenspan, and from feedback from several industrial users of Markdown, including Jeff Atwood, Vincent Marti, and Neil Williams.