Can have_selector Match Only Part of an Elements Class String?

[DanS]

I would like to find a way to check for the existence of a class on a
<td> element in a calendar. The <td>s are the days of the calendar
and can have various classes assigned. I want to test if a day has
the 'past' class and don't care (in this test) if other classes
(weekend, today, etc) are present. So far I've only been able to use
have_selector('td', :class=>'some class') if the class string matches
exactly. Is there an easy way to match if the class 'past' is just
present in the string? (wildcards, regex, whatever)
Making the test match the string exactly feels like it makes the test
too complicated and loose its focus on the single thing I want to
test.
Thanks to anyone who can offer advice on this.
Dan Steinicke