An implementation of MultiMarkdown in C, using a PEG grammar - a fork of jgm's peg-markdown. No longer under active development - see MMD 5.
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What is this?

This is an implementation of John Gruber's markdown in C. It uses a parsing expression grammar (PEG) to define the syntax. This should allow easy modification and extension. It currently supports output in HTML, LaTeX, or groff_mm formats, and adding new formats is relatively easy.

It is pretty fast. A 179K text file that takes 5.7 seconds for (v. 1.0.1) to parse takes less than 0.2 seconds for this markdown. It does, however, use a lot of memory (up to 4M of heap space while parsing the 179K file, and up to 80K for a 4K file). (Note that the memory leaks in earlier versions of this program have now been plugged.)

Both a library and a standalone program are provided.

peg-markdown is written and maintained by John MacFarlane (jgm on github), with significant contributions by Ryan Tomayko (rtomayko). It is released under the GPL; see LICENSE for details.


This program is written in portable ANSI C. It requires glib2. Most *nix systems will have this installed already. The build system requires GNU make.

The other required dependency, Ian Piumarta's peg/leg PEG parser generator, is included in the source directory. It will be built automatically. (However, it is not as portable as peg-markdown itself, and seems to require gcc.)

To make the 'markdown' executable:


(Or, on some systems, gmake.) Then, for usage instructions:

./markdown --help

To run John Gruber's Markdown 1.0.3 test suite:

make test

The test suite will fail on one of the list tests. Here's why. encloses "item one" in the following list in <p> tags:

1.  item one
    * subitem
    * subitem

2.  item two

3.  item three

peg-markdown does not enclose "item one" in <p> tags unless it has a following blank line. This is consistent with the official markdown syntax description, and lets the author of the document choose whether <p> tags are desired.


peg-markdown supports extensions to standard markdown syntax. These can be turned on using the command line flag -x or --extensions. -x by itself turns on all extensions. Extensions can also be turned on selectively, using individual command-line options. To see the available extensions:

./markdown --help-extensions

The --smart extension provides "smart quotes", dashes, and ellipses.

The --notes extension provides a footnote syntax like that of Pandoc or PHP Markdown Extra.

Using the library

The library exports two functions:

GString * markdown_to_g_string(char *text, int extensions, int output_format);
char * markdown_to_string(char *text, int extensions, int output_format);

The only difference between these is that markdown_to_g_string returns a GString (glib's automatically resizable string), while markdown_to_string returns a regular character pointer. The memory allocated for these must be freed by the calling program, using g_string_free() or free().

text is the markdown-formatted text to be converted. Note that tabs will be converted to spaces, using a four-space tab stop. Character encodings are ignored.

extensions is a bit-field specifying which syntax extensions should be used. If extensions is 0, no extensions will be used. If it is 0xFFFFFF, all extensions will be used. To set extensions selectively, use the bitwise & operator and the following constants:

  • EXT_SMART turns on smart quotes, dashes, and ellipses.
  • EXT_NOTES turns on footnote syntax. Pandoc's footnote syntax is used here.

output_format is either HTML_FORMAT, LATEX_FORMAT, or GROFF_MM_FORMAT.

To use the library, include markdown_lib.h. See markdown.c for an example.


It should be pretty easy to modify the program to produce other formats than HTML or LaTeX, and to parse syntax extensions. A quick guide:

  • markdown_parser.leg contains the grammar itself.

  • markdown_output.c contains functions for printing the Element structure in various output formats.

  • To add an output format, add the format to markdown_formats in markdown_lib.h. Then modify print_element in markdown_output.c, and add functions print_XXXX_string, print_XXXX_element, and print_XXXX_element_list. Also add an option in the main program that selects the new format. Don't forget to add it to the list of formats in the usage message.

  • To add syntax extensions, define them in the PEG grammar (markdown_parser.leg), using existing extensions as a guide. New inline elements will need to be added to Inline =; new block elements will need to be added to Block =. (Note: the order of the alternatives does matter in PEG grammars.)

  • If you need to add new types of elements, modify the keys enum in markdown_peg.h.

  • By using &{ } rules one can selectively disable extensions depending on command-line options. For example, &{ extension(EXT_SMART) } succeeds only if the EXT_SMART bit of the global syntax_extensions is set. Add your option to markdown_extensions in markdown_lib.h, and add an option in markdown.c to turn on your extension.

  • Note: Avoid using [^abc] character classes in the grammar, because they cause problems with non-ascii input. Instead, use: ( !'a' !'b' !'c' . )