Tried the regex, no matches found.
But was this just targeted at the definition or the text?
e.g. will
<p class="p6 sgc-2">October 11</p>
be changed to
<p class="p6.bold">October 11</p>
The problem is each file has a different meaning for "sgc-2".
And your regex looks for a . but there is none.
What I think I need to do is in every file with the
sgc-2 {font-weight: bold}
definition, to change
" sgc-2">"
to
".bold">"
I don't know if the "Selected files" allows me to choose files by criteria or just manually.