Selective restriction for attributes based on element values.

Hi,
I have an element <startTag> with attribute as type. The value of the 
attribute type can be numeric or alphabetic (with or without limits 
and/or patterns) depending on what was the value of the element. The 
value of the element would be string always. How do I apply such a 
restriction for the attribute ?

Examples :
   <startTag type="x">op1</startTag>
   <startTag type="0">op2</startTag>

Regards,
Nags.