String::Mask is a library I created not for a specific use case, but
simply b/c it had a certain logical sense to it. However, to this day
I have not conceived of any need for it. I'm wondering if anyone else
can think of a case where it might be useful.

I created a QED file which explains it pretty well:

  http://github.com/rubyworks/strmask/blob/master/qed/strmask.rdoc