extensible Markdown library and tool in "pure OCaml"
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Since 2.0.0.alpha (10 October 2018), some instructions here might be out of date. Pull requests are welcome!

OMD: extensible Markdown library and tool in OCaml

OMD provides two things:

  1. the command-line tool omd, which takes some Markdown and converts it to HTML or Markdown.

    Use omd -help for more information on how to use it.

  2. the library for OCaml contains several modules:

    • the module Omd contains most functions a user will need for basic Markdown manipulation.
    • the modules Omd_parser, Omd_lexer, Omd_backend, Omd_representation and Omd_utils basically implement what their names say:
      • Omd_parser implements the parser (the most complex part).
      • Omd_lexer implements a (basic) lexer.
      • Omd_backend implements 3 backends:
        1. HTML: default backend.
        2. Markdown: sometimes it's useful to show that the fix-point is easily reachable.
        3. S-expression: it's mainly used for debugging.
      • Omd_representation declares the datatypes used in Omd. It also provides some functions to work on those datatypes.
      • Omd_utils provides some useful tools that are not very specific to the OMD-specific datatypes.

OMD aims at implementing the "original Markdown specs" with a few Github Flavour Markdown characteristics. OMD is also meant to be more "sane" than other Markdown parsers from the semantics point of view: if something bothers you from the semantics point of view, please open an issue on Github.


OMD assumes its input is US-ASCII or UTF-8 encoded.


OMD is implemented in OCaml, therefore it needs it to be compiled. OCaml 4.00.1 and then 4.01.0 have been used. OMD should be compatible with 3.12.0 as well, if it's not then please open an issue.

The opam package for OMD depends on ocamlfind, which is only used to compile and install OMD.

The root Makefile uses oasis, ocamlbuild and oasis2opam. The Makefile in src/ only use the compilers from the standard distribution of OCaml.

OMD, compiled as a library and/or a tool, doesn't depend on anything other than the OCaml standard library and runtime.


  • to install omd using opam (recommended)

    opam install omd

  • to get the development version of omd

    git clone git://github.com/ocaml/omd.git

  • to compile omd

    • without oasis nor ocamlbuild

      cd omd/src && make

    • using oasis and ocamlbuild

      cd omd && make


The recommended version numbers are typefaced in bold. As new releases come out and bugs are discovered, a version can stop being recommended.

Version numbers are trying to follow this scheme: x.y.z, z is is for minor changes, y may include algorithm, interface or editorial policy changes, and x is for deeper changes.

  • 2.0.0.alpha Big refactoring by @nojb. Changes in interface (simplification of code, might affect performance a little). To be tested!

  • 1.3.x might stop checking validity of HTML tag names and accept any XML-parsable tag name.

  • 1.2.5 only fixes a single bug (an ordered list could be transformed into an unordered list)

  • 1.2.4 only fixes a single bug (some spaces were wrongly handled in the HTML parsing)

  • 1.2.2 and 1.2.3 fix a few issues with HTML parsing.

  • 1.2.1 mainly fixes issues with HTML parsing.

  • 1.2.0 introduces options -w and -W. Fixes mostly concern subtle uses of \ns in HTML and Markdown outputs.

  • 1.1.2: fix: some URL-related parsing issues.

  • 1.1.0 and 1.1.1: fix: some HTML-related issues.

  • 1.0.1: fixes some parsing issues, improves output. (2014-10-02)

  • 1.0.0: warning: this release is only partially compatible with previous versions.

  • tags 1.0.0.x precede 1.0.0. Also, tags 1.0.0.x will not be released in OPAM, next release will be 1.0.0. And 1.0.0.x may not be compatible with each other.

  • tag 1.0.0.g: accept HTML blocks which directly follow each other

  • tag 1.0.0.f: fix: accept all XML-compatible attribute names for HTML attributes

  • tag 1.0.0.e: fix backslash-escaping for hash-ending ATX-titles + fix Markdown output for Html_block

  • tag 1.0.0.d: fix (HTML parsing) bugs introduced in 1.0.0.b and 1.0.0.c

  • tag 1.0.0.c: rewrite parser of block HTML to use the updated Omd.t

  • tag 1.0.0.b: rewrite parser of inline HTML to use the updated Omd.t

  • tag 1.0.0.a: upgrade Omd.t for HTML representation

There will not be any newer 0.9.x release although new bugs have been discovered. Thus it's recommended to upgrade to the latest 1.x.y.

  • 0.9.7: introduction of media:end + bug fixes

    If you need to have a version that still has Tag of extension instead of Tag of name * extension and don't want to upgrade, you may use 0.9.3

  • 0.9.6: fix a bug (concerning extensions) introduced by 0.9.4.

  • 0.9.5: bug fix + Tag of extension changed to Tag of name * extension

  • 0.9.4: fixes a bug for the new feature

  • 0.9.3: new feature media:type="text/omd".

    This version is recommended if you do not use that new feature and want to use 0.9.x

  • 0.9.2: not released...

  • older versions: cf. commit log