Negative superscripts do not work

[SuborbitalPigeon]

Trying to format units such as K^-1 doesn't render properly when using K^-1^ just passes the text through. I need to add a space between the unit and the power. Values of superscripts which don't start with - do work however.

[jmooring]

To clarify...

• This works: K ^-1^
• This does not: K^-1^

We have the same problem with subscripts, inserts, etc.

TagKind	This works	This does not work
kindSuperscript	K^1^s	K^-1^
kindSubscript	K~1~	K~-1~
kindInsert	K++1++	K++-1++
kindMark	K==1==	K==-1==
kindDelete	K~~1~~	K~~-1~~

The first character within the delimiters must be a number or a letter. It fails if the first character is punctuation of any type.

cc: @bowman2001

[jmooring]

Goldmark's strikethrough extension has the same problem:
yuin/goldmark#483

And GitHub itself fails as well.

Markdown:

K~1~

K~*1~

Rendered by GitHub:

K1

K~*1~

[SuborbitalPigeon]

To clarify...

Sorry, I was in a bit of a rush.

[jmooring]

From Goldmark's project lead:

This kind of extensions [referring to strikethrough] are implemented as same as the emphasis. CommonMark emphasis algorithm is known for complicated and having many edge cases.

This Markdown:

K _1_

K_1_

K_-1_

Is rendered to this:

<p>K <em>1</em></p>
<p>K_1_</p>
<p>K_-1_</p>

Rendering is the same with the Goldmark playground and GitHub.