Check out http://www.zenspider.com/Languages/Ruby/QuickRef.html#11 for
details on this portion of the regex. It seems to involve look-ahead
capability.

HTH.

Steve Throckmorton wrote:
> 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?