From: Gennady <gfb@...>
Date: 2004-06-11T08:22:48+09:00
Subject: Re: String#scan strangeness

David A. Black wrote:

> 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
> 

Thanks, David. It looks like this is the case. Actually, I solved my 
problem by using the following regexp instead:

[linux.gfbs:71]gfb-ems-session_1> irb
irb(main):001:0> a = "a b c d "
=> "a b c d "
irb(main):002:0> a.scan %r{#{'(\S+\s+)' * 2}}
=> [["a ", "b "], ["c ", "d "]]
irb(main):003:0>

(My actual regexp is much bigger, I just used a simplified form for an 
example)

Gennady.