From: dblack@...
Date: 2007-05-27T12:11:19+09:00
Subject: Re: are MatchData objects mutable?

Hi --

On Sun, 27 May 2007, Mike Steiner wrote:

> A few times in my programs I've tried to modify elements of the array
> returned by .match, and it always causes strange errors later in my code.
>
> For example, if I do:
>
> s = "aaa bbb ccc"
> m = /(.*) (.*) (.*)/.match(s)
> m[1].sub! ( "aa" , "" )
>
> I get strange results, but if I instead do:
>
> s = "aaa bbb ccc"
> m = /(.*) (.*) (.*)/.match(s)
> x = m[1].dup
> x.sub! ( "aa" , "" )
>
> it works fine. What's going on?

Looking at the source (fairly quickly, but I think this is what's
happening), it looks like m[1] calls match_aref, which calls
rb_reg_nth_match, which hands back a different string object each
time.  So doing a sub! on it doesn't have any impact on the string you
get the next time you do the [1] operation.

You can also see this like this, where each [1] string is a different
object:

irb(main):006:0> m = /(abc)/.match("abc")
=> #<MatchData:0x34bf98>
irb(main):007:0> m[1].object_id
=> 1713410
irb(main):008:0> m[1].object_id
=> 1698110
irb(main):009:0> m[1].object_id
=> 1684420

etc.


David

-- 
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