How do I scan starting at the end of a big io object to find a string
that is always closer to then end?

here's very roughly how the contents of the IO object look:

Lots's of stuff... lot's of stuff....lot's of
stuff...."my_string_i_need"...lot's of stuff

In other words, I have a massive IO object that I cannot reverse and I
need to very quickly find a string towards the end (but still fairly
far from the end).  What's the fastest way to do this?

Thanks in advance!