MigrateXMLReader regex is choking on attribute xpath using 'contains()'

MigrateXMLReader parses the elementyQuery xpath to validate its location during iteration (next). Unfortunately I've run into some xpath which the regex is choking on. (This is for a Taxonomy term migration which is why it is so specific.)

$item_xpath = "/catalog_titles/catalog_title/category[@scheme[contains(.,'genre')]]";

The first pass for the element query results in:

Array
(
    [0] => Array
        (
            [0] => /catalog_titles/catalog_title/category[@scheme[contains(.,'genre')]]
        )

    [1] => Array
        (
            [0] => catalog_titles/catalog_title/category
        )

    [2] => Array
        (
            [0] => [@scheme[contains(.,'genre')]]
        )

)

However, the following attribute search:
preg_match_all('|^\[@([^=]+)=[\'"](.*)[\'"]\]$|', $attribute_query, $matches);
on:
[@scheme[contains(.,'genre')]] returns no results.

I'm not the xpath whiz but I'll try to take some stabs at this. Unless someone reports that this is resolved in the dev version???