[ruby-core:33102] Re: Suggestion for MatchData#first and #last

On Mon, Nov 8, 2010 at 9:34 AM, NARUSE, Yui <[e-mail]> wrote:
> 2010/11/8 Robert Klemme <[e-mail]>:
>> Btw, I just notice this
>>
>> irb(main):016:0> /(?<f>fo+).*(\g<f>)/ =~ "foooobarfoo"
>> => 0
>> irb(main):017:0> $1
>> => "foo"
>> irb(main):018:0> $2
>> => nil
>> irb(main):019:0> f
>> => "foo"
>> irb(main):020:0> RUBY_VERSION
>> => "1.9.1"
>> irb(main):021:0> RUBY_PATCHLEVEL
>> => 429
>>
>> I thought named group would have the same capturing semantics as
>> unnamed groups but apparently the leading named group is not
>> considered as capturing. s the behavior shown above intended?
>
> When you use named capture, you can't use normal parentheses.
> It becomes (? .. ).

I think this is not true, because as long as I do not use \g I can
access the group via name *and* via index:

irb(main):004:0> /(?<a>fo+)/ =~ "foobar"
=> 0
irb(main):005:0> $1
=> "foo"
irb(main):006:0> a
=> "foo"

That's also in line with what the doc says
http://www.geocities.jp/kosako3/oniguruma/doc/RE.txt

Or did you mean "when using *calls* to named captures"?  Because
that's where the difference comes in.

> So you should like following:
>
> irb(main):001:0> /(?<f>fo+).*(?<g>\g<f>)/ =~ "foooobarfoo"
> => 0
> irb(main):002:0> $1
> => "foo"
> irb(main):003:0> $2
> => "foo"

This still feels wrong: named captures work quite differently than
unnamed captures then.  When I start out with this:

irb(main):015:0> /(?<f>fo+).*\g<f>/ =~ "foooobarfoo"
=> 0
irb(main):016:0> $1
=> "foo"
irb(main):017:0> $2
=> nil
irb(main):018:0> f
=> "foo"

this is basically the same as

irb(main):023:0> /(fo+).*\1/ =~ "foooobarfoo"
=> 0
irb(main):024:0> $1
=> "foo"
irb(main):025:0> $2
=> nil

Now I add parentheses

irb(main):026:0> /(?<f>fo+).*(\g<f>)/ =~ "foooobarfoo"
=> 0
irb(main):027:0> $1
=> "foo"
irb(main):028:0> $2
=> nil

and expect the same to happen as with unnamed groups:

irb(main):029:0> /(fo+).*(\1)/ =~ "foooobarfoo"
=> 0
irb(main):030:0> $1
=> "foo"
irb(main):031:0> $2
=> "foo"

You can see that now $2 is set.  I just expect named captures to work
the same.  But it does not work even if I embed the backref in a
longer regexp:

irb(main):032:0> /(?<f>fo+).*(r\g<f>)/ =~ "foooobarfoo"
=> 0
irb(main):033:0> $1
=> "foo"
irb(main):034:0> $2
=> nil

I believe $2 should now be "rfoo" because that's what happens with
unnamed captures:

irb(main):035:0> /(fo+).*(r\1)/ =~ "foooobarfoo"
=> 0
irb(main):036:0> $1
=> "foo"
irb(main):037:0> $2
=> "rfoo"

Kind regards

robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/