>>> using asterisks
before **strong** after

<<<
<p>before <strong>strong</strong> after</p>
>>> using underscores
before __strong__ after

<<<
<p>before <strong>strong</strong> after</p>
>>> unmatched asterisks
before ** after

<<<
<p>before ** after</p>
>>> unmatched underscores
before __ after

<<<
<p>before __ after</p>
>>> multiple spans in one text
a **one** b __two__ c

<<<
<p>a <strong>one</strong> b <strong>two</strong> c</p>
>>> multi-line
before **first
second** after

<<<
<p>before <strong>first
second</strong> after</p>