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
_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