# Regexp backtracking issues in 1.8

**URL:** <https://www.ruby-forum.com/t/regexp-backtracking-issues-in-1-8/87994>\
**Category:** Ruby\
**Created:** [March 7, 2007, 11:56am UTC](https://www.ruby-forum.com/t/regexp-backtracking-issues-in-1-8/87994 "2007-03-07T11:56:38Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Vasile\_G](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/cc9497/32.png) [@Vasile\_G](https://www.ruby-forum.com/u/Vasile_G)\
**Post date:** [March 7, 2007, 11:56am UTC](https://www.ruby-forum.com/t/regexp-backtracking-issues-in-1-8/87994/1 "2007-03-07T11:56:38Z")

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Basically Ruby 1.8 fails to backtrack correctly a number of regular  
expressions. The simplest one is probably:

irb(main):036:0\> /a(ba|.)\*?a/.match(‘axba’)  
=\> nil

There is more to it, and since I had to write it down for my students,  
I’m just going to link to the gory details:  
[http://www.cs.umd.edu/~gaburici/rubyre.html](http://www.cs.umd.edu/~gaburici/rubyre.html)

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**Author:** ![Vasile\_G](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/cc9497/32.png) [@Vasile\_G](https://www.ruby-forum.com/u/Vasile_G)\
**Post date:** [March 7, 2007, 1:14pm UTC](https://www.ruby-forum.com/t/regexp-backtracking-issues-in-1-8/87994/2 "2007-03-07T13:14:14Z")

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On 3/7/07, Vasile G. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> –  
> Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/).

I just followed the link to find this jewel:

Ruby 1.8 has a rather buggy regexp engine. Some constructs using  
alternation followed by a Kleene star operator will not behave as  
expected, or as they do in Python or Perl.

very funny I think

Cheers  
Robert

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<div class="post-metadata">

**Author:** ![Vasile\_G](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/cc9497/32.png) [@Vasile\_G](https://www.ruby-forum.com/u/Vasile_G)\
**Post date:** [March 7, 2007, 1:26pm UTC](https://www.ruby-forum.com/t/regexp-backtracking-issues-in-1-8/87994/3 "2007-03-07T13:26:17Z")

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Vasile G. schrieb:

> Basically Ruby 1.8 fails to backtrack correctly a number of regular  
> expressions. The simplest one is probably:
> 
> irb(main):036:0\> /a(ba|.)\*?a/.match(‘axba’)  
> =\> nil
> 
> There is more to it, and since I had to write it down for my students,  
> I’m just going to link to the gory details:  
> [http://www.cs.umd.edu/~gaburici/rubyre.html](http://www.cs.umd.edu/~gaburici/rubyre.html)

It is an Error in Ruby 1.8. In Ruby 1.9 (with Oniguruma) it works  
correct  
(“ruby-445” is a special naming of myself for some experiments):

> > > > > Example \>\>\>\>\>

C:\Dokumente und Einstellungen\wolfgang\Desktop\>type regexerr.rb  
p “axba”.match(/a(ba|.)\*?a/)  
C:\Dokumente und Einstellungen\wolfgang\Desktop\>ruby -v  
ruby 1.8.5 (2006-08-25) [i386-mswin32]

C:\Dokumente und Einstellungen\wolfgang\Desktop\>ruby regexerr.rb  
nil

C:\Dokumente und Einstellungen\wolfgang\Desktop\>ruby-445 regexerr.rb  
#MatchData:0x2ac7ab8

C:\Dokumente und Einstellungen\wolfgang\Desktop\>ruby-445 -v  
ruby 1.9.0 (2006-10-28) [i386-mingw32]

> > > > > EoE \>\>\>\>\>

Wolfgang Nádasi-Donner

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<div class="post-metadata">

**Author:** ![Vasile\_G](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/cc9497/32.png) [@Vasile\_G](https://www.ruby-forum.com/u/Vasile_G)\
**Post date:** [March 8, 2007, 1:42am UTC](https://www.ruby-forum.com/t/regexp-backtracking-issues-in-1-8/87994/4 "2007-03-08T01:42:43Z")

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Vasile G. wrote:

> Basically Ruby 1.8 fails to backtrack correctly a number of regular  
> expressions. The simplest one is probably:
> 
> irb(main):036:0\> /a(ba|.)\*?a/.match(‘axba’)  
> =\> nil
> 
> There is more to it, and since I had to write it down for my students,  
> I’m just going to link to the gory details:  
> [http://www.cs.umd.edu/~gaburici/rubyre.html](http://www.cs.umd.edu/~gaburici/rubyre.html)

I’ve posted a lot of very short test cases with varying quantifiers. The  
results are interesting:  
[http://nikolasco.livejournal.com/374793.html](http://nikolasco.livejournal.com/374793.html)

Regards,  
-Nikolas