Function preprocess_latex 

pub fn preprocess_latex(content: &str) -> String

/\\\[(.*?)\\\]/ -> $$...$$ (single-line bracket math) /\\\[([\s\S]*?)\\\]/ -> $$...$$ (block bracket math) /\\\((.*?)\\\)/ -> $$...$$ (paren math) /(^|[^\\])\$(.+?)\$/ -> $1$$$2$ (dollar math)