Restricting string content

Hi,

I am developing a schema that has an element whose content needs to be
restricted to valid c identifiers.  Basically the problem I have run
into is there doesn't seem to be a mechanism in schemas to restrict the
content of an element by saying that a string cannot be a certain word.
i.e. <ValidCIdent>char</ValidCIdent>
-should not be allowed as it is a reserved word, while 'chara' would be
allowed.

It doesn't seem that patterns would achieve this very well, but I may be
wrong.

What is the best way to do this?  

Thank you in advance

Mike