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

Subject: Re: [Boost-users] boost::xpressive - Regex stack space exhausted
From: Eric Niebler (eric_at_[hidden])
Date: 2010-07-22 08:33:44

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?

>>> 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.

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