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Initial release
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# MD4C Readme
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* Home: http://github.com/mity/md4c
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* Wiki: http://github.com/mity/md4c/wiki
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* Issue tracker: http://github.com/mity/md4c/issues
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MD4C stands for "Markdown for C" and that's exactly what this project is about.
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## What is Markdown
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In short, Markdown is the markup language this `README.md` file is written in.
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The following resources can explain more if you are unfamiliar with it:
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* [Wikipedia article](http://en.wikipedia.org/wiki/Markdown)
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* [CommonMark site](http://commonmark.org)
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## What is MD4C
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MD4C is Markdown parser implementation in C, with the following features:
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* **Compliance:** Generally, MD4C aims to be compliant to the latest version of
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[CommonMark specification](http://spec.commonmark.org/). Currently, we are
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fully compliant to CommonMark 0.31.
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* **Extensions:** MD4C supports some commonly requested and accepted extensions.
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See below.
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* **Performance:** MD4C is [very fast](https://talk.commonmark.org/t/2520).
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* **Compactness:** MD4C parser is implemented in one source file and one header
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file. There are no dependencies other than standard C library.
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* **Embedding:** MD4C parser is easy to reuse in other projects, its API is
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very straightforward: There is actually just one function, `md_parse()`.
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* **Push model:** MD4C parses the complete document and calls few callback
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functions provided by the application to inform it about a start/end of
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every block, a start/end of every span, and with any textual contents.
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* **Portability:** MD4C builds and works on Windows and POSIX-compliant OSes.
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(It should be simple to make it run also on most other platforms, at least as
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long as the platform provides C standard library, including a heap memory
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management.)
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* **Encoding:** MD4C by default expects UTF-8 encoding of the input document.
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But it can be compiled to recognize ASCII-only control characters (i.e. to
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disable all Unicode-specific code), or (on Windows) to expect UTF-16 (i.e.
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what is on Windows commonly called just "Unicode"). See more details below.
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* **Permissive license:** MD4C is available under the [MIT license](LICENSE.md).
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## Using MD4C
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### Parsing Markdown
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If you need just to parse a Markdown document, you need to include `md4c.h`
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and link against MD4C library (`-lmd4c`); or alternatively add `md4c.[hc]`
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directly to your code base as the parser is only implemented in the single C
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source file.
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The main provided function is `md_parse()`. It takes a text in the Markdown
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syntax and a pointer to a structure which provides pointers to several callback
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functions.
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As `md_parse()` processes the input, it calls the callbacks (when entering or
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leaving any Markdown block or span; and when outputting any textual content of
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the document), allowing application to convert it into another format or render
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it onto the screen.
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### Converting to HTML
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If you need to convert Markdown to HTML, include `md4c-html.h` and link against
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MD4C-HTML library (`-lmd4c-html`); or alternatively add the sources `md4c.[hc]`,
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`md4c-html.[hc]` and `entity.[hc]` into your code base.
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To convert a Markdown input, call `md_html()` function. It takes the Markdown
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input and calls the provided callback function. The callback is fed with
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chunks of the HTML output. Typical callback implementation just appends the
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chunks into a buffer or writes them to a file.
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## Markdown Extensions
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The default behavior is to recognize only Markdown syntax defined by the
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[CommonMark specification](http://spec.commonmark.org/).
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However, with appropriate flags, the behavior can be tuned to enable some
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extensions:
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* With the flag `MD_FLAG_COLLAPSEWHITESPACE`, a non-trivial whitespace is
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collapsed into a single space.
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* With the flag `MD_FLAG_TABLES`, GitHub-style tables are supported.
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* With the flag `MD_FLAG_TASKLISTS`, GitHub-style task lists are supported.
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* With the flag `MD_FLAG_STRIKETHROUGH`, strike-through spans are enabled
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(text enclosed in tilde marks, e.g. `~foo bar~`).
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* With the flag `MD_FLAG_PERMISSIVEURLAUTOLINKS` permissive URL autolinks
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(not enclosed in `<` and `>`) are supported.
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* With the flag `MD_FLAG_PERMISSIVEEMAILAUTOLINKS`, permissive e-mail
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autolinks (not enclosed in `<` and `>`) are supported.
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* With the flag `MD_FLAG_PERMISSIVEWWWAUTOLINKS` permissive WWW autolinks
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without any scheme specified (e.g. `www.example.com`) are supported. MD4C
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then assumes `http:` scheme.
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* With the flag `MD_FLAG_LATEXMATHSPANS` LaTeX math spans (`$...$`) and
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LaTeX display math spans (`$$...$$`) are supported. (Note though that the
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HTML renderer outputs them verbatim in a custom tag `<x-equation>`.)
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* With the flag `MD_FLAG_WIKILINKS`, wiki-style links (`[[link label]]` and
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`[[target article|link label]]`) are supported. (Note that the HTML renderer
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outputs them in a custom tag `<x-wikilink>`.)
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* With the flag `MD_FLAG_UNDERLINE`, underscore (`_`) denotes an underline
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instead of an ordinary emphasis or strong emphasis.
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Few features of CommonMark (those some people see as mis-features) may be
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disabled with the following flags:
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* With the flag `MD_FLAG_NOHTMLSPANS` or `MD_FLAG_NOHTMLBLOCKS`, raw inline
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HTML or raw HTML blocks respectively are disabled.
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* With the flag `MD_FLAG_NOINDENTEDCODEBLOCKS`, indented code blocks are
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disabled.
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## Input/Output Encoding
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The CommonMark specification declares that any sequence of Unicode code points
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is a valid CommonMark document.
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But, under a closer inspection, Unicode plays any role in few very specific
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situations when parsing Markdown documents:
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1. For detection of word boundaries when processing emphasis and strong
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emphasis, some classification of Unicode characters (whether it is
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a whitespace or a punctuation) is needed.
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2. For (case-insensitive) matching of a link reference label with the
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corresponding link reference definition, Unicode case folding is used.
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3. For translating HTML entities (e.g. `&`) and numeric character
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references (e.g. `#` or `ಫ`) into their Unicode equivalents.
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However note MD4C leaves this translation on the renderer/application; as
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the renderer is supposed to really know output encoding and whether it
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really needs to perform this kind of translation. (For example, when the
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renderer outputs HTML, it may leave the entities untranslated and defer the
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work to a web browser.)
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MD4C relies on this property of the CommonMark and the implementation is, to
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a large degree, encoding-agnostic. Most of MD4C code only assumes that the
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encoding of your choice is compatible with ASCII. I.e. that the codepoints
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below 128 have the same numeric values as ASCII.
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Any input MD4C does not understand is simply seen as part of the document text
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and sent to the renderer's callback functions unchanged.
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The two situations (word boundary detection and link reference matching) where
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MD4C has to understand Unicode are handled as specified by the following
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preprocessor macros (as specified at the time MD4C is being built):
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* If preprocessor macro `MD4C_USE_UTF8` is defined, MD4C assumes UTF-8 for the
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word boundary detection and for the case-insensitive matching of link labels.
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When none of these macros is explicitly used, this is the default behavior.
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* On Windows, if preprocessor macro `MD4C_USE_UTF16` is defined, MD4C uses
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`WCHAR` instead of `char` and assumes UTF-16 encoding in those situations.
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(UTF-16 is what Windows developers usually call just "Unicode" and what
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Win32API generally works with.)
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Note that because this macro affects also the types in `md4c.h`, you have
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to define the macro both when building MD4C as well as when including
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`md4c.h`.
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Also note this is only supported in the parser (`md4c.[hc]`). The HTML
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renderer does not support this and you will have to write your own custom
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renderer to use this feature.
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* If preprocessor macro `MD4C_USE_ASCII` is defined, MD4C assumes nothing but
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an ASCII input.
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That effectively means that non-ASCII whitespace or punctuation characters
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won't be recognized as such and that link reference matching will work in
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a case-insensitive way only for ASCII letters (`[a-zA-Z]`).
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## Documentation
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The API of the parser is quite well documented in the comments in the `md4c.h`.
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Similarly, the markdown-to-html API is described in its header `md4c-html.h`.
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There is also [project wiki](http://github.com/mity/md4c/wiki) which provides
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some more comprehensive documentation. However note it is incomplete and some
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details may be somewhat outdated.
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## FAQ
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**Q: How does MD4C compare to other Markdown parsers?**
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**A:** Some other implementations combine Markdown parser and HTML generator
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into a single entangled code hidden behind an interface which just allows the
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conversion from Markdown to HTML. They are often unusable if you want to
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process the input in any other way.
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Second, most parsers (if not all of them; at least within the scope of C/C++
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language) are full DOM-like parsers: They construct abstract syntax tree (AST)
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representation of the whole Markdown document. That takes time and it leads to
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bigger memory footprint.
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Building AST is completely fine as long as you need it. If you don't, there is
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a very high chance that using MD4C will be substantially faster and less hungry
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in terms of memory consumption.
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Last but not least, some Markdown parsers are implemented in a naive way. When
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fed with a [smartly crafted input pattern](test/pathological_tests.py), they
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may exhibit quadratic (or even worse) parsing times. What MD4C can still parse
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in a fraction of second may turn into long minutes or possibly hours with them.
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Hence, when such a naive parser is used to process an input from an untrusted
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source, the possibility of denial-of-service attacks becomes a real danger.
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A lot of our effort went into providing linear parsing times no matter what
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kind of crazy input MD4C parser is fed with. (If you encounter an input pattern
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which leads to a sub-linear parsing times, please do not hesitate and report it
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as a bug.)
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**Q: Does MD4C perform any input validation?**
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**A:** No. And we are proud of it. :-)
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CommonMark specification states that any sequence of Unicode characters is
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a valid Markdown document. (In practice, this more or less always means UTF-8
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encoding.)
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In other words, according to the specification, it does not matter whether some
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Markdown syntax construction is in some way broken or not. If it's broken, it
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won't be recognized and the parser should see it just as a verbatim text.
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MD4C takes this a step further: It sees any sequence of bytes as a valid input,
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following completely the GIGO philosophy (garbage in, garbage out). I.e. any
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ill-formed UTF-8 byte sequence will propagate to the respective callback as
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a part of the text.
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If you need to validate that the input is, say, a well-formed UTF-8 document,
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you have to do it on your own. The easiest way how to do this is to simply
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validate the whole document before passing it to the MD4C parser.
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## License
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MD4C is covered with MIT license, see the file `LICENSE.md`.
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## Links to Related Projects
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Ports and bindings to other languages:
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* [commonmark-d](https://github.com/AuburnSounds/commonmark-d):
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Port of MD4C to D language.
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* [markdown-wasm](https://github.com/rsms/markdown-wasm):
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Port of MD4C to WebAssembly.
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* [PyMD4C](https://github.com/dominickpastore/pymd4c):
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Python bindings for MD4C
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Software using MD4C:
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* [imgui_md](https://github.com/mekhontsev/imgui_md):
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Markdown renderer for [Dear ImGui](https://github.com/ocornut/imgui)
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* [MarkDown Monolith Assembler](https://github.com/1Hyena/mdma):
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A command line tool for building browser-based books.
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* [QOwnNotes](https://www.qownnotes.org/):
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A plain-text file notepad and todo-list manager with markdown support and
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ownCloud / Nextcloud integration.
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* [Qt](https://www.qt.io/):
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Cross-platform C++ GUI framework.
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* [Textosaurus](https://github.com/martinrotter/textosaurus):
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Cross-platform text editor based on Qt and Scintilla.
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* [8th](https://8th-dev.com/):
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Cross-platform concatenative programming language.
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