Since question types we are developed used to do heavy string processing, we are developed a class to make it more easy. For now it distributed as part of abstract POASQuestion type (well, it's in Plugins but waiting for approval, so you may not be able to see it). But you may want to use it too or include in the Moodle.
The main benefit of this class is that it allows to safely use square brackets with unicode strings, and it converts to the PHP string when necessary. It also have "ord" function, that is sadly missing from textlib.
I attached a file with class to the message. Please comment on it's interface and usefullness.