searching backwards in a string

[Paul Rubin]

I have a string and want to search backwards for a regexp, starting at
a given index, like if you search backwards in a text editor.  Is
there a way to do that with re?  The docs aren't clear about what's
supposed to happen if the ending position to the pat.search method is
less than the starting position, but I tried it and it doesn't seem
to do what I want.  Any advice appreciated--thanks.