I have an XML document that I would like to remove/filter whole nodes
(and their child nodes) if an attribute is not contained/matched. I
would like to output a new XML document, not modifying the original.
Can anyone recommend XSLT for this or a programmatic way if it's
faster?

<item look="a;b">
    <item look="a;b">
        <subitem/>
        <subitem/>
        <item look="a">
            <subitem/>
            </subitem/>
        </item>
    </item>
<item>

I would like to apply an expression to the above document, so any
<item> element that does not have a "b" in the look attribute is
removed. I would like the result XML document to be:

<item look="a;b">
    <item look="a;b">
        <subitem/>
        <subitem/>
    </item>
<item>

Thanks.