Awesome

NOTE -- Showdown on GitHub

Please note that I, Corey, am not the author of Showdown. Rather, I found it some time back at http://attacklab.net/showdown/ and wanted to see it available on GitHub.

All credit and praise for authoring this library should go to John Fraser.

Oh, and John Gruber of course.

That said, I have recently see some GitHub forking activity and a pull request. As such, I will endeavor to ordain myself as a maintainer of Showdown, at least as it exists on GitHub.

Apologies for any confusion or perceived misinformation.

Cheers,
Corey

Showdown -- A JavaScript port of Markdown

Redistributable under a BSD-style open source license. See license.txt for more information.

What's it for?

Developers can use Showdown to:

Add in-browser preview to existing Markdown apps

Showdown's output is (almost always) identical to markdown.pl's, so the server can reproduce exactly the output that the user saw. (See below for exceptions.)
Add Markdown input to programs that don't support it

Any app that accepts HTML input can now be made to speak Markdown by modifying the input pages's HTML. If your application lets users edit documents again later, then they won't have access to the original Markdown text. But this should be good enough for many uses -- and you can do it with just a two-line onsubmit function!
Add Markdown input to closed-source web apps

You can write bookmarklets or userscripts to extend any standard textarea on the web so that it accepts Markdown instead of HTML. With a little more hacking, the same can probably be done with many rich edit controls.
Build new web apps from scratch

A Showdown front-end can send back text in Markdown, HTML or both, so you can trade bandwidth for server load to reduce your cost of operation. If your app requires JavaScript, you won't need to do any Markdown processing on the server at all. (For most uses, you'll still need to sanitize the HTML before showing it to other users -- but you'd need to do that anyway if you're allowing raw HTML in your Markdown.)

Browser Compatibility

Showdown has been tested successfully with:

Firefox 1.5 and 2.0
Internet Explorer 6 and 7
Safari 2.0.4
Opera 8.54 and 9.10
Netscape 8.1.2
Konqueror 3.5.4

In theory, Showdown will work in any browser that supports ECMA 262 3rd Edition (JavaScript 1.5). The converter itself might even work in things that aren't web browsers, like Acrobat. No promises.

Known Differences in Output

In most cases, Showdown's output is identical to that of Perl Markdown v1.0.2b7. What follows is a list of all known deviations. Please email me if you find more.

This release uses the HTML parser from Markdown 1.0.2b2, which means it fails Inline HTML (Advanced).text from the Markdown test suite:
```
<div>
<div>
unindented == broken
</div>
</div>
```
Showdown doesn't support the markdown="1" attribute:
```
<div markdown="1">
     Markdown does *not* work in here.
</div>
```
This is half laziness on my part and half stubbornness. Markdown is smart enough to process the contents of span- level tags without screwing things up; shouldn't it be able to do the same inside block elements? Let's find a way to make markdown="1" the default.
You can only nest square brackets in link titles to a depth of two levels:
```
[[fine]](http://www.attacklab.net/)
[[[broken]]](http://www.attacklab.net/)
```
If you need more, you can escape them with backslashes.

When sublists have paragraphs, Showdown produces equivalent HTML with a slightly different arrangement of newlines:

+ item

     - subitem

       The HTML has a superfluous newline before this
       paragraph.

     - subitem

       The HTML here is unchanged.

     - subitem

       The HTML is missing a newline after this
       list subitem.

Markdown.pl creates empty title attributes for inline-style images:
```
Here's an empty title on an inline-style
![image](http://w3.org/Icons/valid-xhtml10).
```
I tried to replicate this to clean up my diffs during testing, but I went too far: now Showdown also makes empty titles for reference-style images:
```
Showdown  makes an empty title for
reference-style ![images][] too.

[images]: http://w3.org/Icons/valid-xhtml10
```
With crazy input, Markdown will mistakenly put <strong> or <em> tags in URLs:
```
<a href="<*Markdown adds em tags in here*>">
   improbable URL
</a>
```
Showdown won't. But still, don't do that.

Credits

Origins
- John Fraser:
  Author of Showdown
- John Gruber:
  Author of Markdown
Maintenance
- Corey Innis:
  GitHub project maintainer
- Remy Sharp:
  CommonJS-compatibility and more