regex/example/snippets/partial_regex_grep.cpp

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <boost/regex.hpp>

// match some kind of html tag:
boost::regex e("<[^>]*>");
// count how many:
unsigned int tags = 0;
// saved position of partial match:
char* next_pos = 0;

bool grep_callback(const boost::match_results<char*>& m)
{
   if(m[0].matched == false)
   {
      // save position and return:
      next_pos = m[0].first;
   }
   else
      ++tags;
   return true;
}

void search(std::istream& is)
{
   char buf[4096];
   next_pos = buf + sizeof(buf);
   bool have_more = true;
   while(have_more)
   {
      // how much do we copy forward from last try:
      unsigned leftover = (buf + sizeof(buf)) - next_pos;
      // and how much is left to fill:
      unsigned size = next_pos - buf;
      // copy forward whatever we have left:
      memmove(buf, next_pos, leftover);
      // fill the rest from the stream:
      unsigned read = is.readsome(buf + leftover, size);
      // check to see if we've run out of text:
      have_more = read == size;
      // reset next_pos:
      next_pos = buf + sizeof(buf);
      // and then grep:
      boost::regex_grep(grep_callback,
                        buf,
                        buf + read + leftover,
                        e,
                        boost::match_default | boost::match_partial);
   }
}

int main(int argc, char* argv[])
{
   if(argc > 1)
   {
      for(unsigned int i = 1; i < argc; ++i)
      {
         std::ifstream fs(argv[i]);
         if(fs.bad()) continue;
         search(fs);
      }
   }
   else
   {
      std::string one("<META NAME=\"keywords\" CONTENT=\"regex++, regular expressions, regular expression library, C++\">");
      std::string what;
      while(what.size() < 10000)
      {
         what.append(one);
         what.append(13, ' ');
      }
      std::stringstream ss;
      ss.str(what);
      search(ss);
   }
   std::cout << "total tag count was " << tags << std::endl;
   return 0;
}