\DeclareMathOperator{rk}{rank} and \newcommand{Lim}{…} don't work in LaTeX (with default catcodes), but the other two (with or without braces around the command name) work just fine in LaTeX. — LSpice34 mins ago
I wrote in the answer:
> Notice that in MathJax (unlike LaTeX) all of these syntaxes work: \DeclareMathOperator{rk}{rank}\DeclareMathOperator{\rk}{rank} and \DeclareMathOperator\rk{rank}. It is similar for \newcommand and \renewcommand (where \newcommand{\Lim}{....}, \newcommand\Lim{....} and \newcommand{Lim}{....} are valid). This should be kept in mind when creating a regex for the blacklist.
I should go and check which of them actually work in LaTeX.
