# Are MatchData objects mutable?

**URL:** <https://www.ruby-forum.com/t/are-matchdata-objects-mutable/105197>\
**Category:** Ruby\
**Created:** [May 27, 2007, 1:59am UTC](https://www.ruby-forum.com/t/are-matchdata-objects-mutable/105197 "2007-05-27T01:59:04Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![mschwab](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/59ef9b/32.png) [@mschwab](https://www.ruby-forum.com/u/mschwab)\
**Post date:** [May 27, 2007, 1:59am UTC](https://www.ruby-forum.com/t/are-matchdata-objects-mutable/105197/1 "2007-05-27T01:59:04Z")

</div>

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?

Mike S.

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<div class="post-metadata">

**Author:** ![mschwab](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/59ef9b/32.png) [@mschwab](https://www.ruby-forum.com/u/mschwab)\
**Post date:** [May 30, 2007, 8:07pm UTC](https://www.ruby-forum.com/t/are-matchdata-objects-mutable/105197/2 "2007-05-30T20:07:09Z")

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Sorry, but now I’m completely confused. Arrays don’t have a .sub!  
method,  
and m[1] is a string. And I can reference m[1], m[2], etc. in any order  
and  
get the same strings each time.

Why are the object id’s of m[1] different each time you reference it (in  
your example)?

Mike S.

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<div class="post-metadata">

**Author:** ![mschwab](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/59ef9b/32.png) [@mschwab](https://www.ruby-forum.com/u/mschwab)\
**Post date:** [May 27, 2007, 5:12am UTC](https://www.ruby-forum.com/t/are-matchdata-objects-mutable/105197/3 "2007-05-27T05:12:19Z")

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Hi –

On Sun, 27 May 2007, Mike S. wrote:

> 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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<div class="post-metadata">

**Author:** ![mschwab](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/59ef9b/32.png) [@mschwab](https://www.ruby-forum.com/u/mschwab)\
**Post date:** [May 30, 2007, 11:25pm UTC](https://www.ruby-forum.com/t/are-matchdata-objects-mutable/105197/4 "2007-05-30T23:25:40Z")

</div>

Now I get it. Thanks!

Mike S.

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<div class="post-metadata">

**Author:** ![mschwab](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/59ef9b/32.png) [@mschwab](https://www.ruby-forum.com/u/mschwab)\
**Post date:** [May 30, 2007, 10:18pm UTC](https://www.ruby-forum.com/t/are-matchdata-objects-mutable/105197/5 "2007-05-30T22:18:26Z")

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On Thu, May 31, 2007 at 03:06:06AM +0900, Mike S. wrote:

> Sorry, but now I’m completely confused. Arrays don’t have a .sub! method,  
> and m[1] is a string. And I can reference m[1], m[2], etc. in any order and  
> get the same strings each time.
> 
> Why are the object id’s of m[1] different each time you reference it (in  
> your example)?

Because m isn’t an Array, it’s a MatchData object.

$ irb1.8  
irb(main):001:0\> m = /(b+)/.match(“abbbc”)  
=\> #MatchData:0xb7d40020

MatchData has a [] method, but that doesn’t mean it has to behave like  
an  
Array. If m[1] were to return the same string each time, it would have  
to  
cache its own answers, or build an array containing all the captures  
(which  
would be wasteful in those cases where they’re not used).

Regards,

Brian.