requesting list of approved characters in extension names

[cszoke-sangoma]

apologies in advance, this is a bit of an old issue, and we've made corrections to prevent the root cause that allowed this to occur, so we probably won't be able to reproduce to provide any further logging or debugging.

we discovered this error while troubleshooting extensions being unable to register to freeswitch.
[ERR] switch_xml.c:1695 Error[[error near line 2]: markup outside of root element]

It was determined to be caused by this oddball character that was inserted instead of an apostrophe, by a third party application.
[customer] => Carpenter�s
[external_caller_id_name] => Carpenter�s

In hope of preventing any such issue in the future, we have put, and are planning on putting an additional fail-safe in place to prevent characters from being sent to freeswitch in extension registration attempts, but are unsure of what isn't accepted. Conservatively, there are probably dozens of characters that are unacceptable, so determining what isn't allowed seems a bit too difficult. Rather than trying to hunt them all down, is there any way to provide us with what punctuation is explicitly supported in extension names, and callerid, or any other variable that accepts ascii?