# Zero-width positive "look-behind"?

**URL:** <https://www.ruby-forum.com/t/zero-width-positive-look-behind/186590>\
**Category:** Ruby\
**Created:** [April 9, 2010, 10:40am UTC](https://www.ruby-forum.com/t/zero-width-positive-look-behind/186590 "2010-04-09T10:40:45Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![nmotter](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/f0a364/32.png) [@nmotter](https://www.ruby-forum.com/u/nmotter)\
**Post date:** [April 9, 2010, 10:40am UTC](https://www.ruby-forum.com/t/zero-width-positive-look-behind/186590/1 "2010-04-09T10:40:45Z")

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> “tttttattttt”.match(/ttt(?=a)/) # [Rubular: ttt(?=a)](http://rubular.com/r/21Zo6JP09f)  
> =\> [“ttt”]  
> “tttttattttt”.match(/(?=a)ttt/) # [Rubular: (?=a)ttt](http://rubular.com/r/d1VSUwwZUu)  
> =\> nil

Why? And how can I get this effect?

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**Author:** ![nmotter](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/f0a364/32.png) [@nmotter](https://www.ruby-forum.com/u/nmotter)\
**Post date:** [April 9, 2010, 12:10pm UTC](https://www.ruby-forum.com/t/zero-width-positive-look-behind/186590/2 "2010-04-09T12:10:54Z")

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On 04/09/2010 10:40 AM, Nathan wrote:

> > “tttttattttt”.match(/ttt(?=a)/) # [Rubular: ttt(?=a)](http://rubular.com/r/21Zo6JP09f)  
> > =\> [“ttt”]  
> > “tttttattttt”.match(/(?=a)ttt/) # [Rubular: (?=a)ttt](http://rubular.com/r/d1VSUwwZUu)  
> > =\> nil
> 
> Why? And how can I get this effect?

It seems you have just demonstrated how to get the effect you described.

For more info:

[http://www.geocities.jp/kosako3/oniguruma/doc/RE.txt](http://www.geocities.jp/kosako3/oniguruma/doc/RE.txt)

Note: you need Ruby 1.9.

Kind regards

robert

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<div class="post-metadata">

**Author:** ![nmotter](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/f0a364/32.png) [@nmotter](https://www.ruby-forum.com/u/nmotter)\
**Post date:** [April 9, 2010, 2:14pm UTC](https://www.ruby-forum.com/t/zero-width-positive-look-behind/186590/3 "2010-04-09T14:14:25Z")

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Ah (?\<=a)ttt is what I was after. Thanks for the link.