cmark-gfm/README.md

CommonMark
==========

CommonMark is a rationalized version of Markdown syntax,
with a [spec][the spec] and BSD3-licensed reference
implementations in C and JavaScript.

[Try it now!](http://spec.commonmark.org/dingus.html)

The implementations
-------------------

The C implementation provides both a shared library (`libcmark`) and a
standalone program `cmark` that converts CommonMark to HTML.  It is
written in standard C99 and has no library dependencies.  The parser is
very fast, on par with [sundown](https://github.com/vmg/sundown).  Some
benchmarks (on an ancient Thinkpad running Intel Core 2 Duo at 2GHz,
measured using `time` and parsing a ~500K book, the English version of
[*Pro Git*](https://github.com/progit/progit/tree/master/en) by Scott
Chacon and Ben Straub):

|Implementation | Time  |  Factor|
|---------------|-------|--------|
| Markdown.pl   | 5.162s|   286.8|
| PHP Markdown  | 1.021s|    56.7|
| commonmark.js | 0.292s|    16.2|
| peg-markdown  | 0.279s|    15.5|
| marked        | 0.239s|    13.3|
| discount      | 0.090s|     5.0|
| **cmark**     | 0.020s|     1.1|
| sundown       | 0.018s|     1.0|

It is easy to use `libcmark` in python or ruby code:  see `wrapper.py`
and `wrapper.rb` in the repository for simple examples.

The JavaScript implementation is a single JavaScript file, with
no dependencies, that can be linked to in an HTML page.  Here
is a simple usage example:

``` javascript
var reader = new commonmark.DocParser();
var writer = new commonmark.HtmlRenderer();
var parsed = reader.parse("Hello *world*");
var result = writer.render(parsed);
```

A node package is also available; it includes a command-line tool called
`commonmark`.

**A note on security:**
Neither implementation attempts to sanitize link attributes or
raw HTML.  If you use these libraries in applications that accept
untrusted user input, you must run the output through an HTML
sanitizer to protect against
[XSS attacks](http://en.wikipedia.org/wiki/Cross-site_scripting).

Installing (C)
--------------

Building the C program (`cmark`) and shared library (`libcmark`)
requires [cmake] and [re2c], which is used to generate `scanners.c` from
`scanners.re`.  (Note that [re2c] is only a build dependency for
developers, since `scanners.c` can be provided in a released source
tarball.)

If you have GNU make, you can simply `make`, `make test`, and `make
install`.  This calls [cmake] to create a `Makefile` in the `build`
directory, then uses that `Makefile` to create the executable and
library.

For a more portable method, you can use [cmake] manually. [cmake] knows
how to create build environments for many build systems.  For example,
on FreeBSD:

    mkdir build
    cd build
    cmake ..  # optionally: -DCMAKE_INSTALL_PREFIX=path
    make      # executable will be create as build/src/cmake
    make test
    make install

Or, to create Xcode project files on OSX:

    mkdir build
    cd build
    cmake -G Xcode ..
    make
    make test
    make install

Tests can also be run manually on any executable `$PROG` using:

    perl runtests.pl spec.txt $PROG

The GNU Makefile also provides a few other targets for developers.
To test the shared library via a python wrapper:

    make testlib

To run a "fuzz test" against ten long randomly generated inputs:

    make fuzztest

To run a test for memory leaks using valgrind:

    make leakcheck

To make a release tarball:

    make tarball

Installing (JavaScript)
-----------------------

The JavaScript library can be installed through `npm`:

    npm install commonmark

To build the JavaScript library as a single standalone file:

    browserify --standalone commonmark js/lib/index.js -o js/commonmark.js

Or fetch a pre-built copy from
<http://spec.commonmark.org/js/commonmark.js>`.

To run tests for the JavaScript library:

    make testjs

or

    node js/test.js

The spec
--------

[The spec] contains over 500 embedded examples which serve as conformance
tests.  To run the tests for `cmark`, do `make test`.  To run them for
another Markdown program, say `myprog`, do `make test PROG=myprog`.  To
run the tests for `commonmark.js`, do `make testjs`.

[The spec]:  http://jgm.github.io/CommonMark/spec.html

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](http://daringfireball.net/projects/markdown/syntax) 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.

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](http://jgm.github.io/CommonMark/spec.html#backslash-escapes),
    not just the symbols with special meanings in Markdown. I found
    that it was just too hard to remember which symbols could be
    escaped.

-   It introduces an [alternative syntax for hard line
    breaks](http://jgm.github.io/CommonMark/spec.html#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](http://jgm.github.io/CommonMark/spec.html#links).

-   The rule for HTML blocks differs, though in most real cases it
    shouldn't make a difference. (See
    [here](http://jgm.github.io/CommonMark/spec.html#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 is also makes parsing much easier, avoiding
    expensive backtracking.

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

        > this is 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 [here](http://jgm.github.io/CommonMark/spec.html#motivation).
    I 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. (I could give numerous examples
    of perfectly natural looking lists that nearly every existing
    implementation flubs up.)

-   The spec stipulates that two blank lines break out of all list
    contexts.  This is an attempt to deal with issues that often come up
    when someone wants to have two adjacent lists, or a list followed by
    an indented code block.

-   Changing bullet characters, or changing from bullets to numbers or
    vice versa, starts a new list. I 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](http://jgm.github.io/CommonMark/spec.html#fenced-code-blocks) are supported, delimited by either
    backticks (` ``` `) or tildes (` ~~~ `).

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](http://github.com/jgm/pandoc)) and the first markdown parsers
based on PEG grammars
([peg-markdown](http://github.com/jgm/peg-markdown),
[lunamark](http://github.com/jgm/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](http://johnmacfarlane.net/babelmark2/).  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.

Contributing
------------

There is a [forum for discussing
CommonMark](http://talk.commonmark.org); you should use it instead of
github issues for questions and possibly open-ended discussions.
Use the [github issue tracker](http://github.com/jgm/CommonMark/issues)
only for simple, clear, actionable issues.



[cmake]: http://www.cmake.org/download/
[pandoc]: http://johnmacfarlane.net/pandoc/
[re2c]: http://re2c.org