>>> image
![][foo]

[foo]: http://foo.com/foo.png

<<<
<p><img src="http://foo.com/foo.png" alt="" /></p>
>>> alternate text
![alternate text][foo]

[foo]: http://foo.com/foo.png

<<<
<p><img src="http://foo.com/foo.png" alt="alternate text" /></p>
>>> title
![][foo]

[foo]: http://foo.com/foo.png "optional title"

<<<
<p><img src="http://foo.com/foo.png" alt="" title="optional title" /></p>
>>> invalid alt text
![`alt`][foo]

[foo]: http://foo.com/foo.png "optional title"

<<<
<p><img src="http://foo.com/foo.png" alt="alt" title="optional title" /></p>
>>> shortcut reference image
![foo]

[foo]: http://foo.com/foo.png
<<<
<p><img src="http://foo.com/foo.png" alt="foo" /></p>