Matching nodes which contain one attribute with value

How do I setup a template which matches only those nodes which contain exactly ONE attribute
whose value in turn starts with the pattern "aaa"?

So the following nodes should be matched:

<ns1:somenode myattr="aaabbbccc">...</ns1:somenode>

or

<ns1:somenode myattr:ns2="aaabbbccc">...</ns1:somenode>

but NOT these nodes:

<ns1:somenode myattr="aaabbbccc" secondattr="blah">...</ns1:somenode>
or
<ns1:somenode myattr="bbbaaaccc">...</ns1:somenode>

Ben