Question

I would like to pass the variable from a function to the main scope which I’m calling, I’m trying to do like I use to do in C but it returns nothing.

I want to be able to output and deal with it after the return of the function

#include "StdAfx.h"
#include <regex>
#include <iostream>
#include <string>
#include <conio.h>
using namespace std;

std::tr1::match_results<std::string::const_iterator> match(std::string& regex, const std::string& ip,std::tr1::match_results<std::string::const_iterator> res)
{
   const std::tr1::regex pattern(regex.c_str());
   bool valid = std::tr1::regex_match(ip, res, pattern);
   std::cout << ip << " \t: " << (valid ? "valid" : "invalid") << std::endl;
   cout << "FIRST RES FOUND: " << res[1] << endl;  
   return res;
}

int main()
{

   string regex = "(\\d{1,3}):(\\d{1,3}):(\\d{1,3}):(\\d{1,3})";
   string ip = "49:22:33:444";
   std::tr1::match_results<std::string::const_iterator> res;
   match(regex,ip.c_str(), res);

   cout << "Result >" << res[1] << "< " << endl;


   _getch(); return 0;
}

When I compile and run, The output is: “FIRST RES FOUND: 49
Result ><“

It’s probably a really simple solution but what do I have to do to set it for my main can read it correctly as in: “Result >49<“

Thanks in advance. 🙂

Answer 1

Option 1: Use references:

void match(string& regex, const string& ip, tr1::match_results<string::const_iterator> & res)
{
   const tr1::regex pattern(regex.c_str());
   bool valid = tr1::regex_match(ip, res, pattern);
   cout << ip << " \t: " << (valid ? "valid" : "invalid") << endl;
   cout << "FIRST RES FOUND: " << res[1] << endl;
}

Option 2: Return the result by value and store it:

tr1::match_results<string::const_iterator> match(string& regex, const string& ip)
{
    tr1::match_results<string::const_iterator> res;
    // ...
    return res;
}

int main()
{
    // ...
    tr1::match_results<string::const_iterator> res = match(regex, ip);
}

On a separate note, there should be absolutely no need for all the c_str() calls, as <regex> has a perfectly functional std::string interface. Check the documentation for details, you just have to get a couple of typenames right.

---

Edit: Here are some basic examples on using std::string. There are equivalent constructions for std::wstring, char* and wchar_t*, but std::strings should be the most useful one.

Since <regex> support is still patchy, you should consider the TR1 and Boost alternatives, too; I provide all three and you can pick one:

namespace ns = std;          // for <regex>
namespace ns = std::tr1;     // for <tr1/regex>
namespace ns = boost;        // for <boost/regex.hpp>

ns::regex r("");
ns::smatch rxres;            // 's' for 'string'

std::string data = argv[1];  // the data to be matched

// Fun #1: Search once

if (!ns::regex_search(data, rxres, r))
{
    std::cout << "No match." << std::endl;
    return 0;
}


// Fun #2:  Iterate over all matches

ns::sregex_iterator rt(data.begin(), data.end(), r), rend;

for ( ; rt != rend; ++rt)
{
    // *rt is the entire match object

    for (auto it = rt->begin(), end = rt->end(); it != end; ++it)
    {
        // *it is the current capture group; the first one is the entire match

        std::cout << "   Match[" << std::distance(rt->begin(), it) << "]: " << *it << ", length " << it->length() << std::endl;
    }
}

Don’t forget to handle exceptions of type ns::regex_error.