_underscore_, *asterisk*, _one two_, *three four*, _a_, *b*

**strong** and *em* and **strong** and *em*

_line
line
line_

this_is_not_an_emphasis

an empty emphasis __ ** is not an emphasis

*mixed **double and* single asterisk** spans

<p><em>underscore</em>, <em>asterisk</em>, <em>one two</em>, <em>three four</em>, <em>a</em>, <em>b</em></p>
<p><strong>strong</strong> and <em>em</em> and <strong>strong</strong> and <em>em</em></p>
<p><em>line
line
line</em></p>
<p>this_is_not_an_emphasis</p>
<p>an empty emphasis __ ** is not an emphasis</p>
<p>*mixed *<em>double and</em> single asterisk** spans</p>