From Namespaces in XML
NCName ::= (Letter | '_') (NCNameChar)*
NCNameChar ::= Letter | Digit | '.' | '-' | '_' | CombiningChar | Extender
From Python Reference Manual (2.3 Identifiers and keywords)
identifier ::= (letter|'_') (letter | digit | '_')*
Default set of anames
ANAME ::= ('_') (letter | digit | '_')*