I am not a Latexpurt either, but I think nearly anything can be in the [ of a command, it would be hard to find a suitable heuristic that worked enough of the time to avoid the reverse bug (arguments not highlighted :)
The solution is probably as pointed out on the Scintilla report, add a wordlist that specified no-option commands, but I think there are a lot of them, see [here](https://www.sharelatex.com/learn/List_of_Greek_letters_and_math_symbols) for some (don't forget to follow the links at the bottom for more) including `\in`. Since the report is nearly a year old I guess anybody who can do it isn't interested in doing it. --- Reply to this email directly or view it on GitHub: https://github.com/geany/geany/issues/859#issuecomment-170292164
