Names
XML 1.0 names must adhere to the following lexical conventions:
Begin with a letter, ideograph, _, or : character.
After the first character, be composed only of
letters, digits, ., -,
_, and :
characters.
In this context, a letter is any Unicode character that
matches the Letter production
from the XML 1.0 EBNF grammar at the end of this chapter.
See the XML 1.1 EBNF grammar production for Name to see which characters are
permitted within XML 1.1 names.
According to the XML 1.0 specification, the : character may be used freely within names, although the character is now officially reserved as part of the "Namespaces in XML" recommendation. Even if a document does not use namespaces, the colon should still not be used within identifiers to maintain compatibility with namespace-aware parsers. See the Section 21.3.4 in this chapter for more information about how namespace-aware identifiers are formed.
Names should also avoid starting with the three-letter sequence X, M, L (in any case combination), unless specifically sanctioned by an XML specification.