rewrite handler handles references to contextual values incorrect

Context:
A URL in the form of "http://somehost.org/path/a/b" should be rewritten to "http://somehost.org/newpath?param1=a&param2=b"

The following filter should perform this rewrite:

<expression-filter name="rewrite-path" expression="regex('^/path/([^/]+)/(.*)/?$') -> rewrite('/newpath'); set(attribute='%{QUERY_STRING}', value='param1=$1&amp;param2=$2'); done()"/>

Problem:
The output is (according to access-log);

somehost.org "GET /newpath?param1=b 

• param1 is assigned to the value of $2
• param2 is dropped completly