on of the boost::regex_search result contains an extra character

[jszirani]

The issue was found in some custom URL parsing code, I am not sure this is a known issue.

I've isolated the issue to this tiny bit of code:

static boost::regex pattern(R"((&?([^&=]+)(=?([^&]*))))", 
                            boost::regex_constants::extended | boost::regex_constants::optimize);
boost::smatch matches;
std::string testString = "SERVICE=WMTS&TileCol=1";
if (boost::regex_search(testString.cbegin(), testString.cend(), matches, pattern)) {
  for (unsigned int i = 0; i < matches.size(); ++i) {
    std::cout << i << ": " << std::string(matches[i].first, matches[i].second) << std::endl;
  }
}

this will print

0: SERVICE=WMTS 1: SERVICE=WMTS 2: SERVICE 3: =WMTS 4: =WMTS

but I think the last item (number 4) should not have the '=' character present, you can check the regular expression on https://regex-vis.com/ that the = is out of group 4

if you switch to std regex

static std::regex pattern(R"((&?([^&=]+)(=?([^&]*))))", 
                          std::regex_constants::extended | std::regex_constants::optimize);
std::smatch matches;
std::string testString = "SERVICE=WMTS&TileCol=1";
if (std::regex_search(testString.cbegin(), testString.cend(), matches, pattern)) {
  for (size_t i = 0; i < matches.size(); ++i) {
    std::cout << i << ": " << std::string(matches[i].first, matches[i].second) << std::endl;
  }
}

it will print

0: SERVICE=WMTS 1: SERVICE=WMTS 2: SERVICE 3: =WMTS 4: WMTS

[jzmaddock]

Well... I think we're correct here: you've asked for POSIX extended regular expressions which means "leftmost longest" and that rule applies to sub-expressions as well. In this case the matcher finds the match you were expecting first (and this is what is returned had you specified a perl regular expression), then rather than stopping at that point as a perl matcher would do, it backtracks looking for something "better". The first backtrack tries matching with the =? part matching zero times, and low and behold the [^&]* then matches to the next & just like before, only this time it starts further left and so is considered "better". Hope that makes sense!