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 10:01:44

On Thu, Jul 22, 2010 at 2:33 PM, Eric Niebler <eric_at_[hidden]> wrote:
> On 7/22/2010 5:29 AM, Pavol Supa wrote:
>> 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));
>
> Wait a minute. What does the data you're trying to match look like? Do
> you know that xdigit only matches a single hex character? Your data
> should look something like:
>
> 00 11 22 33 44 55 66 77 88 99 aa bb cc dd ee ff 00 11 ....
>
> Is that right?

i used data just without spaces, 000000000000...00000

the data are most often without spaces, so i decided meanwhile to use
the workaround (it doesn't throw):
expr = * repeat<2,2> (xdigit)
but if i move the inner expression to a function, it starts throwing
(at similar length), i.e.:
sregex byte() { return repeat<2,2> (xdigit) }
sregex expr = * byte();

>
>>>> 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.
>
> That doesn't sound right. This could be a bug. I'll look at this today.
>

i tried to debug the parsing, but it looked ok - when there is a
non-trivial expression inside (as mentioned above), it goes into
recursion, about 12 functions on callstack, and it eats whole
(default) stack at those mentioned quantities

> --
> Eric Niebler
> BoostPro Computing
> http://www.boostpro.com
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