# How to use regexp assertion

**URL:** <https://www.ruby-forum.com/t/how-to-use-regexp-assertion/135448>\
**Category:** Ruby\
**Created:** [April 22, 2008, 9:57am UTC](https://www.ruby-forum.com/t/how-to-use-regexp-assertion/135448 "2008-04-22T09:57:54Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![smellcode](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/f0a364/32.png) [@smellcode](https://www.ruby-forum.com/u/smellcode)\
**Post date:** [April 22, 2008, 9:57am UTC](https://www.ruby-forum.com/t/how-to-use-regexp-assertion/135448/1 "2008-04-22T09:57:54Z")

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how to use regexp assertion  
like this in perl  
/some\_pattern(?=exp)/

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**Author:** ![smellcode](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/f0a364/32.png) [@smellcode](https://www.ruby-forum.com/u/smellcode)\
**Post date:** [April 22, 2008, 11:06am UTC](https://www.ruby-forum.com/t/how-to-use-regexp-assertion/135448/2 "2008-04-22T11:06:52Z")

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On Apr 22, 2008, at 00:57 , smellcode wrote:

> how to use regexp assertion  
> like this in perl  
> /some\_pattern(?=exp)/

> **[Ruby | zenspider.com | by ryan davis](https://www.zenspider.com/ruby/)**
>
> Ruby facts, opinions, source code, links, and stuff.

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<div class="post-metadata">

**Author:** ![smellcode](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/f0a364/32.png) [@smellcode](https://www.ruby-forum.com/u/smellcode)\
**Post date:** [April 22, 2008, 11:21am UTC](https://www.ruby-forum.com/t/how-to-use-regexp-assertion/135448/3 "2008-04-22T11:21:01Z")

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3q, but i mean  
how to use (?\<!exp) and (?\<=exp)  
in irb  
undefined (?..) sequence:  
/\d+(?\<=\w)/[http://data.movie.xunlei.com/movie/39224](http://data.movie.xunlei.com/movie/39224)

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<div class="post-metadata">

**Author:** ![smellcode](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/f0a364/32.png) [@smellcode](https://www.ruby-forum.com/u/smellcode)\
**Post date:** [April 22, 2008, 11:40am UTC](https://www.ruby-forum.com/t/how-to-use-regexp-assertion/135448/4 "2008-04-22T11:40:01Z")

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Posted by Jeremy McAnally (Guest)  
on 03.02.2008 07:02

Install the oniguruma gem.

gem install oniguruma

It requires the C lib (installable from ports), but is installed by  
default with Ruby 1.9. It handles lookbehind.

I found this

has any method to do it more native??