Restricting text in mixed content element

I am using XML Spy.

I have a mixed-content element.  I would like to restrict the text in the 
element.  Specifically, I would like to exclude the square brackets.

I tried defining a datatype that restricts xs:string, then extending that to 
the element in question.  That seems to work except when an occurrence of the 
element contains not only text, but also some of the other elements that are 
permitted.

Am I misunderstanding something, or is there a bug in XML Spy?

Richard R. Liu
eBS - Publishing
UBS AG
Hochstr. 16, 2149
CH-4053 Basel
Switzerland
Tel. +41 (0)61 288-7689