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Date: Thu, 30 Jul 2009 12:13:14 -0400
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Subject: Re: [PHP-DEV] Alternative mbstring implementation using ICU
From: gwynne@darkrainfall.org (Gwynne Raskind)

On Jul 30, 2009, at 11:27 AM, Stefan Walk wrote:
>> and that there's nothing you can do with Oniguruma that you can't  
>> also practically do with PCRE (to the best of my knowledge),
> http://www.geocities.jp/kosako3/oniguruma/doc/RE.txt - Paragraph 8,  
> example 2 - specifying the nest level for subroutines/back  
> references doesn't work with pcre. That's one example I know from  
> the top of my head ...


Granted :). But it would be possible to rewrite that example into a  
PCRE regexp, just a bit less efficiently. Subroutine calls *are* one  
thing I like about Oniguruma, but not enough to counter all the other  
arguments in favor of PCRE. Honestly, that sort of extension to the  
syntax verges on kicking regexp out of the domain it was intended for;  
function calls are verging on making regexp trivially Turing-complete.  
If you need that level of matching, it's probably time to consider  
more aggressive string parsing methodology, such as tokenization.  
Regexp is basically a VM (at least as most implementations handle it),  
and on nontrivial patterns will often be slower than other methods.

Certainly that particular example, matching an XML tag, will never  
deal with all the esoteric cases. You can approach perfect matching as  
pattern complexity approaches infinite (i.e. lim complexity->inf  
correctness = inf), but the pattern is only realistically useful in  
specific cases where you can be guaranteed a valid input stream. And  
supposing it doesn't match, there's no reasonable way to determine  
*why*.

Also, more to the point, Oniguruma hasn't been updated in two years  
and counting, which is enough to count it as unmaintained. PCRE is  
still seeing releases on a regular basis.

-- Gwynne