>>> single asterisks
before *em* after

<<<
<p>before <em>em</em> after</p>
>>> single underscores
before _em_ after

<<<
<p>before <em>em</em> after</p>
>>> double asterisks
before **strong** after

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

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

<<<
<p>before *after</p>
>>> unmatched underscore
before _after

<<<
<p>before _after</p>
>>> multiple spans in one text
a *one* b _two_ c

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

<<<
<p>before <em>first
second</em> after</p>
>>> not processed when surrounded by spaces
a * b * c _ d _ e

<<<
<p>a * b * c _ d _ e</p>
>>> strong then emphasis
**strong***em*

<<<
<p><strong>strong</strong><em>em</em></p>
>>> emphasis then strong
*em***strong**

<<<
<p><em>em</em><strong>strong</strong></p>
>>> emphasis inside strong
**strong *em***

<<<
<p><strong>strong <em>em</em></strong></p>
>>> mismatched in nested
*a _b* c_

<<<
<p><em>a _b</em> c_</p>
>>> in the middle of a word
a_b_c a__b__c a*b*c a**b**c
<<<
<p>a_b_c a__b__c a<em>b</em>c a<strong>b</strong>c</p>
>>> prefixing a word
_a_b __a__b *a*b **a**b
<<<
<p>_a_b __a__b <em>a</em>b <strong>a</strong>b</p>
>>> suffixing a word
a_b_ a__b__ a*b* a**b**
<<<
<p>a_b_ a__b__ a<em>b</em> a<strong>b</strong></p>
>>> spanning words
_a_b c_d_ __a__b c__d__ *a*b c*d* **a**b c**d**
<<<
<p><em>a_b c_d</em> <strong>a__b c__d</strong> <em>a</em>b c<em>d</em> <strong>a</strong>b c<strong>d</strong></p>