Hi Virginia,

I understand that what you want is
ab* | a*b
and you need to write that in a non ambiguous way to be able to write it 
in XML Schema.

So what we need to accept is

a ab abb abbb ...
b ab aab aaab ...


You can write that as

((a, (b* | (a+, b))) | b)

Best Regards,
George
---------------------------------------------------------------------
George Cristian Bina - http://aboutxml.blogspot.com/
<oXygen/> XML Editor, Schema Editor and XSLT Editor/Debugger
www.---.com


Virginia Wiswell wrote:
> I have a similar situation that I'm having trouble with. I have a and b, at
> least one of them has to appear once AND ONLY ONCE. a must come before b.
> The examples in this thread allow more than one occurrence of a or b.
> 
> This is my first attempt at customizing a schema and I'm stuck.
> 
> TIA, Virginia
> 
>> You need to specify the requirements in a little more detail:
>>
>> * are multiple occurrences of a, b, and c allowed?
>>
>> * what constraints do you want to impose on the ordering of the elements?
>>
>> Michael Kay
>> http://www.saxonica.com/
> 
> 
> 
> 

From george@o... Wed Oct 10 08:11:20 2007
Received: from maggie.w3.org (