From: Steve Throckmorton <nntp.m.ttwelve@...>
Date: 2006-07-18T02:35:11+09:00
Subject: mystery regexp

I found this regexp in the docs for the OptionParser class, and I don't 
understand it.  Google hasn't helped, nor has the Pickaxe, so I thought 
I would ask here.  The purpose of the code is to ensure that a file 
extension submitted by the user begins with a period (a dot).

file_ext.sub!(/\A\.(?=.)/, ".")

I understand all except (?=.)  As far as I can tell the regexp works 
fine without it.  I can delete the dot inside the parens, and the code 
still runs.  I can change the = to a - and the code runs, but if I 
substitute a letter or a * instead, it doesn't.

Someone please clue me in here.  What is this thing?