Re: [Boost-users] regarding "boost::bad_expression"

29 Apr 2006

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i am using boost regex to check the systax of my URLs.
if i am giving a long URL then it is giving the following error:-
terminate called after throwing an instance of
 'boost::bad_expression' what():  Memory exhausted
Aborted
There is a runtime santity check inside the regex matcher that results in an 
exception being thrown if the complexity of the attempted match exceed a 
certain limit.  It's there to prevent you from shooting yourself in the foot 
and writing an expression that takes *forever* to match.

It's possible to up the limit that triggers the exception by changing the 
macros in boost/regex/user.hpp, but really you need to look hard at the 
regex you're using and try and optimise it a bit more.

Problems normally occur when you have something that looks like:

(x*)*

In your case I notice you have a tripply recursive loop like this in the 
fragment:

((([a-zA-Z0-9]+-?)?[a-zA-Z0-9]+)+[.]{1}[a-zA-Z0-9]+)+

First off the fragment [.]{1} is just the same as [.] isn't it?

The matcher is seeing this as being to all intents and purposes the same as:

(([a-zA-Z0-9-.]+)+)+

which is what causes the regex to "thrash" when trying to find a match.  I 
assume you want to assert that . and - are always followed by one or more 
[a-zA-Z0-9] characters?

So how about:

([a-zA-Z0-9]|[.-](?=[a-zA-Z0-9]))+

For this whole fragment?

Hope this helps,  John.
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#include<iostream>
#include <boost/regex/v4/regex.hpp>
bool EvaluateRegex(const std::string regularexp,std::string url){
       boost::smatch what;
       boost::regex exp(regularexp,boost::regex::extended);
       boost::regex_search(url,what,exp);
       std::string search=(std::string)what[0];
       if(search.length() == url.length())
               return true;
       else
               return false;
}
int main()
{
       std::string url;
       const std::string URLSYNTAXREGEX =
"((https?|ftp)://)?(((([a-zA-Z0-9]+-?)?[a-zA-Z0-9]+)+[.]{1}[a-zA-Z0-9]+)+([:]{1}((0[0-7]+)|(0[xX][0-9a-fA-F]+)|([0-9]+))){0,1})([?/][-a-zA-Z0-9+&@#/%?=~_|!:,.;
]*)?"; std::cout<<"enter the URL"<<std::endl; std::cin>>url;
if(!EvaluateRegex(URLSYNTAXREGEX,url)) std::cout<<"Malformed
URL"<<std::endl; else std::cout<<"Well formed URL"<<std::endl;
return 0; }
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Re: [Boost-users] regarding "boost::bad_expression"

John Maddock