#include <iostream>
#include <regex>
#include <string>
#include <vector>
using namespace std;
#define debug(exp) do { cout << #exp << ": " << (exp) << endl; } while (0)
int main (int argc, char *argv[])
{
  vector<string> res;
  string pattern (""),
    text ("ilovechina");
  vector<string> dict = { "i", "love", "china", "lovechina", "ilove" };
  vector<string>::iterator dict_it = dict.begin ();
  if (dict_it == dict.end ()) throw invalid_argument (__func__);
  while (true)
    {
      pattern += *dict_it;
      if (++dict_it == dict.end ()) break;
      else pattern += "|";
    }
  debug (pattern);
  regex re (pattern);
  sregex_iterator ri (text.begin (), text.end (), re),
    ri_end;
  for (; ri != ri_end; ++ri)
    {
      ssub_match sm = (*ri)[0];
      if (ri->position (0)) // The near string does not correspond to the dictionary
    {
      debug (ri->position (0));
      debug (ri->length (0));
      debug (string (sm.first, sm.second));
      throw runtime_error ("not in dictionary");
    }
      if (ri->position (0) + ri->length (0) == text.length ()) // successfully delimited all the words in the text
    break;
      res.push_back (sm.str ());
    }
  if (ri != ri_end) throw runtime_error ("not in dictionary");
}

I expect this code to based on the elements of dict, delimit the words in text.
But why does it always skip the first word "i" and goes to the second word "love", outputting 1 as the first value of ri->position (0)?

My output:

pattern: i|love|china|lovechina|ilove
ri->position (0): 1
ri->length (0): 4
string (sm.first, sm.second): love
terminate called after throwing an instance of 'std::runtime_error'
  what():  not in dictionary
Aborted (core dumped)

Probably just a stupid error.