Basic subscript is between ~s:
E=mc~2~.
.
<p>E=mc<sub>2</sub>.</p>
Backslash escapes:
E=mc\~2\~.
.
<p>E=mc~2~.</p>
Spaces and formatting are allowed:
E=mc~2 or *3*~.
.
<p>E=mc<sub>2 or <em>3</em></sub>.</p>
Punctuation is ignored for purposes of determining flankingness:
E=mc~!~.
.
<p>E=mc<sub>!</sub>.</p>
Subscript can't begin or end with a space:
E=mc~ 2~.
E=mc~2 ~.
~ ~
.
<p>E=mc~ 2~.</p>
<p>E=mc~2 ~.</p>
<p>~ ~</p>