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sirthias / pegdown

A pure-Java Markdown processor based on a parboiled PEG parser supporting a number of extensions
http://pegdown.org
Apache License 2.0
1.29k stars 216 forks source link

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:>>> DEPRECATION NOTE <<<:

Although still one of the most popular Markdown parsing libraries for the JVM, pegdown has reached its end of life.

The project is essentially unmaintained with tickets piling up and crucial bugs not being fixed.
pegdown's parsing performance isn't great. In some cases of pathological input runtime can even become exponential, which means that the parser either appears to "hang" completely or abort processing after a time-out.

Therefore pegdown is not recommended anymore for use in new projects requiring a markdown parser.
Instead I suggest you turn to @vsch's flexmark-java, which appears to be an excellent replacement for these reasons:

In case you need support with migrating from pegdown to flexmark-java, @vsch welcomes inquiries in here or here.

Introduction

Maven Central Javadoc

pegdown is a pure Java library for clean and lightweight Markdown processing based on a parboiled PEG parser.

pegdown is nearly 100% compatible with the original Markdown specification and fully passes the original Markdown test suite. On top of the standard Markdown feature set pegdown implements a number of extensions similar to what other popular Markdown processors offer. You can also extend pegdown by your own plugins! Currently pegdown supports the following extensions over standard Markdown:

Note: pegdown differs from the original Markdown in that it ignores in-word emphasis as in

> my_cool_file.txt
> 2*3*4=5

Currently this "extension" cannot be switched off.

Installation

You have two options:

Usage

Using pegdown is very simple: Just create a new instance of a PegDownProcessor and call one of its markdownToHtml methods to convert the given Markdown source to an HTML string. If you'd like to customize the rendering of HTML links (Auto-Links, Explicit-Links, Mail-Links, Reference-Links and/or Wiki-Links), e.g. for adding rel="nofollow" attributes based on some logic you can supply your own instance of a LinkRenderer with the call to markdownToHtml.

You can also use pegdown only for the actual parsing of the Markdown source and do the serialization to the target format (e.g. XML) yourself. To do this just call the parseMarkdown method of the PegDownProcessor to obtain the root node of the Abstract Syntax Tree for the document. With a custom Visitor implementation you can do whatever serialization you want. As an example you might want to take a look at the sources of the ToHtmlSerializer.

Note that the first time you create a PegDownProcessor it can take up to a few hundred milliseconds to prepare the underlying parboiled parser instance. However, once the first processor has been built all further instantiations will be fast. Also, you can reuse an existing PegDownProcessor instance as often as you want, as long as you prevent concurrent accesses, since neither the PegDownProcessor nor the underlying parser is thread-safe.

See http://sirthias.github.com/pegdown/api for the pegdown API documentation.

Plugins

Since parsing and serialisation are two different things there are two different plugin mechanisms, one for the parser, and one for the ToHtmlSerializer. Most plugins would probably implement both, but it is possible that a plugin might just implement the parser plugin interface.

For the parser there are two plugin points, one for inline plugins (inside a paragraph) and one for block plugins. These are provided to the parser using the PegDownPlugins class. For convenience of use this comes with its own builder. You can either pass individual rules to this builder (which is what you probably would do if you were using Scala rules), but you can also pass it a parboiled Java parser class which implements either InlinePluginParser or BlockPluginParser or both. PegDownPlugins will enhance this parser for you, so as a user of a plugin you just need to pass the class to it (and the arguments for that classes constructor, if any). To implement the plugin, you would write a normal parboiled parser, and implement the appropriate parser plugin interface. You can extend the pegdown parser, this is useful if you want to reuse any of its rules.

For the serializer there is ToHtmlSerializerPlugin interface. It is called when a node that the ToHtmlSerializer doesn't know how to process is encountered (i.e. one produced by a parser plugin). Its accept method is passed the node, the visitor (so if the node contains child nodes they can be rendered using the parent) and the printer for the plugin to print to. The accept method returns true if it knew how to handle the node or false if otherwise and the ToHtmlSerializer loops through each plugin breaking when it reaches one that returns true and if it finds none throws an exception like it used to.

As an very simple example you might want to take a look at the sources of the PluginParser test class.

Parsing Timeouts

Since Markdown has no official grammar and contains a number of ambiguities the parsing of Markdown source, especially with enabled language extensions, can be "hard" and result, in certain corner cases, in exponential parsing time. In order to provide a somewhat predictable behavior pegdown therefore supports the specification of a parsing timeout, which you can supply to the PegDownProcessor constructor.

If the parser happens to run longer than the specified timeout period it terminates itself with an exception, which causes the markdownToHtml method to return null. Your application should then deal with this case accordingly and, for example, inform the user.

The default timeout, if not explicitly specified, is 2 seconds.

IDE Support

The excellent idea-markdown plugin for IntelliJ IDEA, RubyMine, PhpStorm, WebStorm, PyCharm and appCode uses pegdown as its underlying parsing engine. The plugin gives you proper syntax-highlighting for markdown source and shows you exactly, how pegdown will parse your texts.

Credits

A large part of the underlying PEG grammar was developed by John MacFarlane and made available with his tool peg-markdown.

License

pegdown is licensed under Apache License 2.0.

Patch Policy

Feedback and contributions to the project, no matter what kind, are always very welcome. However, patches can only be accepted from their original author. Along with any patches, please state that the patch is your original work and that you license the work to the pegdown project under the project’s open source license.