Re: [Boost-users] boost::xpressive - Regex stack space exhausted

Subject: Re: [Boost-users] boost::xpressive - Regex stack space exhausted
From: Pavol Supa (pavol.supa_at_[hidden])
Date: 2010-07-22 05:29:54

On Wed, Jul 21, 2010 at 10:52 PM, Eric Niebler <eric_at_[hidden]> wrote:
> On 7/21/2010 2:39 PM, Pavol Supa wrote:
>> Hi,
>>
>> I need to match a hex-written byte array, optionally separated with
>> spaces. So i tried:
>>
>> boost::xpressive::sregex r = * ( * blank >> repeat<2,2> (xdigit));
>> smatch match;
>> regex_match (input, match, r);
>>
>> when i use input of approx. 150 hex pairs, i get an exception "Regex
>> stack space exhausted" (i use default stack size by Visual studio
>> 2008)
>>
>> This pattern looks quite simple, so I'd like to know, if there is some
>> fundamental problem with this expressions.
>
> Yes. See
> http://www.boost.org/doc/libs/1_43_0/doc/html/xpressive/user_s_guide.html#boost_xpressive.user_s_guide.tips_n_tricks.beware_nested_quantifiers
>
> Not only will this pattern tear through stack, it'll run very slowly.
> Try this instead:
>
> * ( keep(*blank) >> repeat<2,2> (xdigit))

So, i tried it. It throws exceptions at ~230 hexdigit pairs.
I played with "keep"s, the only 'better' combination is

* keep ( (*blank) >> repeat<2,2> (xdigit))

which throws when input has 300 pairs

>
>> I know i can enlarge the stack manually, but i expect the input to be
>> cca 10kB long, depending on user input, so i consider it not being a
>> good solution.
>
> --
> Eric Niebler
> BoostPro Computing
> http://www.boostpro.com
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