What's the limitations of PDO named arguments

Using prepared statements with PDO, I understand it as there's two paths, either ? or :name.

What are the limitations regarding the named parameters? White spaces? Non ASCII-chars?

(I'm well acquainted with the hell of non-ASCII in field开发者_StackOverflow社区 names. So please stick to the topic.)

Those are tokens. Limits are probably A-Z, 0-9 characters, not starting with 0-9.

From that switch starting on the line of 304 I would say it is [a-zA-Z0-9_]