From: dblack@...
Date: 2003-05-24T10:23:22+09:00
Subject: Re: why is "does" missing from this sub!-stitution?

Hi --

On Sat, 24 May 2003, Dave Thomas wrote:

> dblack@superlink.net wrote:
>
> > My first thought by way of explaining the $2 behavior was: there's
> > been no assignment to $2, because the regex has been compiled but the
> > match hasn't happened yet.  But then \2 wouldn't be meaningful....  So
> > I do actually start to wonder why the previous $2 (nil or otherwise)
> > persists so long.
>
> \2 is substituted by .sub (ie, it gets passed in as part of the string,
> and .sub looks for and substitutes for it). #$2 is substituted during
> the initial parse of the string (before .sub is even called)
>
> irb(main):008:0> "abcdef".sub(/(..)/, "#$1")
> => "cdef"
> irb(main):009:0> "abcdef".sub(/(..)/, "#$1")
> => "abcdef"

I'm with you on the 'what', but I'm still curious about the 'why'.
It's true that the replacement string gets parsed before sub is
called.  But I think "abcdef" gets matched against the regex even
before that parse happens.  Which means... if the $1,$2 variables
reflect the results of the most recent match, then one could argue
that by the time the replacement string is parsed, a match has just
taken place and $1,$2 should reflect the results of that match.

So the order of things seems a little odd:

  /(..)/.match("abcdef")        # match operation, $1 set
  "abcdef".sub(/(..)/, "#$1")   # match operation, $1 *not* set
                                  then string parsed,
                                  *then* $1 is set


David

-- 
David Alan Black
home: dblack@superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav