Re: Is this a legal expression?

no.


<xsl:variable name="teststring" 
				expr="
					customer_id[contains(.,$_id)] and
	

the attribute should be select not expr


other than that it looks legal but probably doesn't do what you intend.
it's a boolean expression (because of the ands) so each of the node sets
selected by the sub terms will be coerced to true or false depending
on whether they are empty


<xsl:if test="$teststring=true">

then the final value will be coerced to a string and tested for equality 
against the string resulting in selecting the child elements called true

David




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