Here's the grammar for symbol names in XML:
Now, "Letter" and "Digit" are internationalized things with not terribly surprising contents; "CombiningChar" is mostly accent characters, and "Extender" is mostly just some characters in other alphabets that aren't exactly letters but which occur in the middle of words (lots of them meaning "repeat previous syllable" or "lengthen previous vowel").
But what's interesting here is "." as a legal character for use in XML elements or attributes -- I've just never ever seen that used.
Has anyone here ever seen <foo.bar