In trying to answer the question Writing text into new line when a particular character is found, I have employed Regexp::Grammars. It has long interested me and finally I had reason to learn. I noticed that the description section the author has a LaTeX parser (I am an avid LaTeX user, so this interested me) but it has one odd construct seen here:
<rule: Option> [^][\$&%#_{}~^\s,]+
<rule: Literal> [^][\$&%#_{}~^\s]+
What do the [^] character classes accomplish?