This is because trackdown interprets anything enclosed in a pair of non-escaped `$` as [inline equations](https://github.com/ClaudioZandonella/trackdown/blob/main/R/utils_rich_text.R#L134).
Also, because `\S` consumes a character, trackdown currently only detects inline equations that have at least 3 characters in them.
Considering the following text:
$x$ blah blah ($\sigma$)
trackdown will highlight `$x$ blah blah ($`
I won't have time to do it this week but I can make a pull request the week after to fix it if you want. I could also take the opportunity to simplify/improve some regex.
For example, in the following case:
Blah blah
r foo$x
blah blahr foo$y
.$x$ blah blah ($\sigma$)