code: public Regex not() {
        if (this.pattern.length() > 0) {
           if (this.pattern.charAt(0) == '[' && this.pattern.length() > 1) {
              return re("[^" + this.pattern.substring(1));
           }
           return re("^" + this.pattern);
        }
        return this;
     }
docs: Negates the regex
  
  @return the negated regex