User blog comment:Ubersketch/Section Notation/@comment-35470197-20190327223409/@comment-36957202-20190330152742

I think a possible definition of what you mean is:

$ is any sequence of symbols inbetween brackets,these brackets ( and )

if you find any of these expressions in the solving of one of these string then:

\($r0=$\)

\($rk=$$rk-1\)