Question

I’m using boost::regex_match and I’m trying to find a function to obtain the position where the regular expression stopped matching. I couldn’t find any attribute in the boost::match_results object, but several code snippets in which boost::regex_search is used to show the submatches. Am I on the right path with my implementation or do I’ve to do something different in order to achieve that? My code looks as follows:

int main(int args, char** argv)
{
    boost::match_results<std::string::const_iterator> what;

    if(args == 3) 
    {
        std::string text(argv[1]);
        boost::regex expression(argv[2]);

        std::cout << "Text : " << text << std::endl;
        std::cout << "Regex: " << expression << std::endl;

        if(boost::regex_match(text, what, expression, boost::match_default) != 0) 
        {
            int i = 0;

            for(boost::match_results<std::string::const_iterator>::const_iterator it=what.begin(); it!=what.end(); ++it) 
            {
                std::cout << "[" << (i++) << "] " << it->str() << std::endl;
            }
            std::cout << "Matched!" << std::endl;
        } 
        else 
        {
            std::string::const_iterator start = text.begin();
            std::string::const_iterator end   = text.end();

            while(boost::regex_search(start, end, what, expression)) 
            {
                std::string submatch(what[1].first, what[1].second);
                std::cout << submatch << std::endl;
                start = what[0].second;
            }
            std::cout << "Didn't match!" << std::endl;
        }
    } //if(args == 3)
    else 
    {
        std::cout << "Invalid usage! $> ./boost-regex <text> <regex>" << std::endl;
    }
    return 0;
}

Output:

$> ./boost_regex "We're building it up to burn it down" ".*(build.*)(to.*)(burn.*)"
Text : We're building it up to burn it down
Regex: .*(build.*)(to.*)(burn.*)
[0] We're building it up to burn it down
[1] building it up
[2] to
[3] burn it down
Matched!

$> ./boost_regex "We're building it up to burm it down" ".*(build.*)(to.*)(burn.*)"
Text : We're building it up to burm it down
Regex: .*(build.*)(to.*)(burn.*)
Didn't match!

For the last input I’d like to have the something similar to:

Text : We're building it up to burm it down
Regex: .*(build.*)(to.*)(burn.*)
[0] We're building it up to
[1] building it up
[2] to
Didn't match!

Thanks in advance …

Answer 1

To start with, you have a problem with the regex in your example. Because it can’t match the (burn.*) subgroup, the entire regex fails to match anything, and returns no results. Adding a “?” after (burn.*) would make it so the first two subgroups would match, and the third would not.

I would checkout rubular.com, it’s a great tool for tweaking your regular expressions and watching them work in realtime.

To test if a subgroup did or did not participate in a match, you need to check the sub_match::matched boolean variable in your for loop, it will be true if the subgroup matched. check the documentation for boost::submatch here

Hope this helped, it’s actually my first Stackoverflow post, answer, or comment. 🙂