Regression tests for link backtracking optimizations:

.
[[some unrelated text [link]

[link]: foo
.
<p>[[some unrelated text <a href="foo">link</a></p>
.

.
[[some unrelated text [[link]]

[[link]]: foo
.
<p>[[some unrelated text <a href="foo">[link]</a></p>
.



This is not a valid emphasis, because \n considered a whitespace:

.
**test
**

**
test**

**
test
**
.
<p>**test
**</p>
<p>**
test**</p>
<p>**
test
**</p>
.



The same for strikethrough:

.
~~test
~~

~~
test~~

~~
test
~~
.
<p>~~test
~~</p>
<p>~~
test~~</p>
<p>~~
test
~~</p>
.



Link label has priority over emphasis (not covered by stmd tests):

.
[**link]()**

**[link**]()
.
<p><a href="">**link</a>**</p>
<p>**<a href="">link**</a></p>
.