XML : lookahead negative in xml

I'm working with xml 1.0, actually xsd-schema, so I use regex restrictions, so I have a problem with lookahead negative. As I've read there is no lookahead function in xml 1.0 restrictions.

Preconditions:

1. String starting and ending with no "/";
2. No "//" in the string;
3. String length is 1-16.

So help me to combine regex equivalent to:

  ([-+A-Z\w\s:?().,'](?:[-+A-Z\w\s:?().,']|/(?!/)){1,14}[-+A-Z\w\s:?().,'])