An example from php markdown readme:

.
*[HTML]: Hyper Text Markup Language
*[W3C]:  World Wide Web Consortium
The HTML specification
is maintained by the W3C.
.
<p>The <abbr title="Hyper Text Markup Language">HTML</abbr> specification
is maintained by the <abbr title="World Wide Web Consortium">W3C</abbr>.</p>
.

They can be multiline (see pandoc implementation). Not sure about newlines, but we should at least skip those definitions:

.
*[
foo
bar
]: desc
foo
.
<p>foo</p>
.

They can contain arbitrary markup (see pandoc implementation):

.
*[`]:`]: foo
\`]:\`
.
<p><abbr title="foo">`]:`</abbr></p>
.

Can contain matched brackets:

.
*[[abbr]]: foo
[abbr]
.
<p><abbr title="foo">[abbr]</abbr></p>
.

No empty abbreviations:

.
*[foo]: 
foo
.
<p>*[foo]:
foo</p>
.

Intersecting abbreviations (first should match):

.
*[Bar Foo]: 123
*[Foo Bar]: 456

Foo Bar Foo

Bar Foo Bar
.
<p><abbr title="456">Foo Bar</abbr> Foo</p>
<p><abbr title="123">Bar Foo</abbr> Bar</p>
.

Don't bother with nested abbreviations (yet?):

.
*[JS]: javascript
*[HTTP]: hyper text blah blah
*[JS HTTP]: is awesome
JS HTTP is a collection of low-level javascript HTTP-related modules
.
<p><abbr title="is awesome">JS HTTP</abbr> is a collection of low-level javascript <abbr title="hyper text blah blah">HTTP</abbr>-related modules</p>
.

Don't match the middle of the string:

.
*[foo]: blah
*[bar]: blah
foobar
.
<p>foobar</p>
.