[xsl] XPath 1.0 Expression to Test for 3 Digit Number

[Don Smith <dsmith_lockesmith@xxxxxxxxx>]

I need an XPath 1.0 expression that will test a node and return a 
boolean if the node content is a 3 digit number (i.e., 100-999) and 
false if it's anything else.

The catch is this expression won't 
be in a transformation so I can't use variables or other additional 
constructs. All I get is one expression. I've tried this

*[(string(number(.)) !='NaN')]


and
 obtained the number that I need. But I can't get any sort of compare to
 work that makes the number be more than 99 and less than 1,000.

Thanks

Don