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

On Thu, Jul 22, 2010 at 3:29 AM, Pavol Supa wrote:

On Wed, Jul 21, 2010 at 10:52 PM, Eric Niebler wrote:

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On 7/21/2010 2:39 PM, Pavol Supa wrote:

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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#bo...

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

If you were using Boost.Spirit.Qi, then it should 'just work', that rule in Boost.Spirit.Qi for a simple match like the above Regex version is would be: boost::spirit::qi::rule r = *lexeme[xdigit >> xdigit]; It can also parse and stuff it all into a string, a vector, or whatever, as characters or parse it into integers/shorts/whatever, basically any parsing need can be easily fulfilled, you really should try Boost.Spirit.Qi.