https://www.v2ex.com/t/254523

main.cpp

#include <iostream>
#include <regex>
#include <fstream>
#include <string>
using namespace std;
int main() {

  std::ifstream ifs("sample.in");
  std::string var((std::istreambuf_iterator<char>(ifs)),
                  (std::istreambuf_iterator<char>()));

  const regex r("answer=\"(.+?)\"");
  smatch sm;

  auto params_it = std::sregex_iterator(var.cbegin(), var.cend(), r);
  auto params_end = std::sregex_iterator();

  while (params_it != params_end) {
    auto param = params_it->str();
    std::regex_match(param, sm, r);
    std::cout << sm[1] << std::endl;
    ++params_it;
  }

  return 0;
}

sample.in

<div class="row">
<div class="span10">
<p>Remember, the <strong>only thing</strong> before everything, is to have a dogma. A dogma is a list consists by DOs and DONTanswer="C"s. You make it, stick to it while adapt it to reality, question yourself from time to time: is everything on the list mutuallanswer="A"y exclanswer="B"usive and collectively eanswer="B"xanswer="C中文"haus再来点中文ive?</p>

<p>And there is a meta dogma for dogmas:</p>

<ul>
<li>Avoid emotional tactics</li>
<li>Every kindness counts</li>
<li>Gentleness is the ultimate strength</li>
</ul>

sample.out

$ clang++ -std=c++11 -stdlib=libc++ -O2 -Wall main.cpp
$ ./a.out
C
A
B
B
C中文