I ran one of my xml files through a schema generator and everything generated was what was expected, with the exception of one node:
http://books.xmlschemata.org/relaxng/ch19-77215.html
No spaces or colons. Allows "_" and "-".
You would use this instead of string so that you can validate that the value is limited to what is allowed. It maps well to certain conventions for name/identifier like django's concept of "slug", for instance.
I upvote the person who [\i-[:]][\c-[:]]* translates into English for us.