how to get positions of all found occurences of a boost::regex pattern in a string?

Question

I can't find working code for that, managed only to coin up this snippet, it compiles but gives wrong output.

#include <string>
#include <iostream>
#include <boost/regex.hpp> 
int main() {
  using namespace std;
  string input = "123 apples 456 oranges 789 bananas oranges bananas";
  boost::regex re = boost::regex("[a-z]+");
  boost::match_results<string::const_iterator> what;
  boost::match_flag_type flags = boost::match_default;
  string::const_iterator s = input.begin();
  string::const_iterator e = input.end();
  while (boost::regex_search(s,e,what,re,flags)){
    cout << what.position() << ", ";
    string::difference_type l = what.length();
    string::difference_type p = what.position();
    s += p + l;
  }
}

output is: 4, 5, 5, 1, 1,

but should be: 4, 15, 27, 35, 43,

Answer 1

You're almost right, but failing to account for the fact that cout << what.position() << ", "; will output the matching string's position relative to the end of the last matching string, ie s.

Since s knows exactly where it is in relation to input, this should work:

cout << ((s - input.begin()) + what.position()) << ", ";