From: "David A. Black" <dblack@...>
Date: 2004-06-11T08:12:37+09:00
Subject: Re: String#scan strangeness

Hi --

On Fri, 11 Jun 2004, Gennady wrote:

> Simon Strandgaard wrote:
> 
> > On Thursday 10 June 2004 18:49, Gennady wrote:
> > 
> >>I am just wondering why String#scan "looses" a group in every match. I
> >>would expect the following result:
> > 
> > 
> > when using sub-captures, then #scan returns an array of sub-captures.
> > This does not include capture[0].. which is the full-match.
> > 
> > "abcd".scan(/(.)(.)/)
> > #=> [["a", "b"], ["c", "d"]]
> > 
> > when not using sub-captures at all, then #scan returns only full-matches.
> > 
> > "abcd".scan(/../)
> > #=> ["ab", "cd"]
> > 
> > --
> > Simon Strandgaard
> > 
> 
> In my original irb session capture I have sub-captures, moreover they 
> are nested:
> 
> [linux.gfbs:281]gfb> ruby -v
> ruby 1.6.8 (2003-10-15) [i686-linux]
> [linux.gfbs:282]gfb> irb
> irb(main):001:0> a = "a b c d "
> => "a b c d "
> irb(main):002:0> a.scan %r{((\S+\s+){2,2})}
> ACTUAL   => [["a b ", "b "], ["c d ", "d "]]
> I EXPECT => [["a b ", "a ", "b "], ["c d ", "c ", "d "]]

My understanding is: you've only got two sets of parentheses, so you
can have at most two captures; in other words, (){2} != ()() :-) It's
purely positional: whatever is in the nth set of parentheses from the
left when the matching stops is the nth capture.

It's as if each () is a window which can move through the string but
can only hold one substring.  So the second set of () sort of moves
from left to right:

   (("a ")....)
   ("a "("b "))    # match completed

   Result:  $1 == "a b "
            $2 == "b "


David

-- 
David A. Black
dblack@wobblini.net