How to check if string is a valid XML element name?

Question

I need a regex or a function in PHP that will validate a string to be a good XML element name.

Form w3schools:

XML elements must follow these

Answer 1

The expression below should match valid unicode element names excepting xml. Names that start or end with xml will still be allowed. This passes @toscho's äøñ test. The one thing I could not figure out a regex for was extenders. The xml element name spec says:

[4] NameChar ::= Letter | Digit | '.' | '-' | '_' | ':' | CombiningChar | Extender

[5] Name ::= (Letter | '_' | ':') (NameChar)*

But there's no clear definition for a unicode category or class containing extenders.

^[\p{L}_:][\p{N}\p{L}\p{Mc}.\-|:]*((?