How to match multiple results using std::regex

Question

For example, If I have a string like "first second third forth" and I want to match every single word in one operation to output them one by one.

I just thought that "(\\b\\S*\\b){0,}" would work. But actually it did not.

What should I do?

Here's my code:

#include<iostream>
#include<string>
using namespace std;
int main()
{
    regex exp("(\\b\\S*\\b)");
    smatch res;
    string str = "first second third forth";
    regex_search(str, res, exp);
    cout << res[0] <<" "<<res[1]<<" "<<res[2]<<" "<<res[3]<< endl;
}   

Answer 1

Simply iterate over your string while regex_searching, like this:

{
    regex exp("(\\b\\S*\\b)");
    smatch res;
    string str = "first second third forth";

    string::const_iterator searchStart( str.cbegin() );
    while ( regex_search( searchStart, str.cend(), res, exp ) )
    {
        cout << ( searchStart == str.cbegin() ? "" : " " ) << res[0];  
        searchStart = res.suffix().first;
    }
    cout << endl;
}

Answer 2

This can be done in regex of C++11.

Two methods:

1. You can use () in regex to define your captures(sub expressions).

Like this:

    string var = "first second third forth";

    const regex r("(.*) (.*) (.*) (.*)");  
    smatch sm;

    if (regex_search(var, sm, r)) {
        for (int i=1; i<sm.size(); i++) {
            cout << sm[i] << endl;
        }
    }

See it live: http://coliru.stacked-crooked.com/a/e1447c4cff9ea3e7

2. You can use sregex_token_iterator():

    string var = "first second third forth";
   
    regex wsaq_re("\\s+"); 
    copy( sregex_token_iterator(var.begin(), var.end(), wsaq_re, -1),
        sregex_token_iterator(),
        ostream_iterator<string>(cout, "\n"));
   

See it live: http://coliru.stacked-crooked.com/a/677aa6f0bb0612f0

Answer 3

sregex_token_iterator appears to be the ideal, efficient solution, but the example given in the selected answer leaves much to be desired. Instead, I found some great examples here: http://www.cplusplus.com/reference/regex/regex_token_iterator/regex_token_iterator/

For your convenience, I've copy-pasted the sample code shown by that page. I claim no credit for the code.

// regex_token_iterator example
#include <iostream>
#include <string>
#include <regex>

int main ()
{
  std::string s ("this subject has a submarine as a subsequence");
  std::regex e ("\\b(sub)([^ ]*)");   // matches words beginning by "sub"

  // default constructor = end-of-sequence:
  std::regex_token_iterator<std::string::iterator> rend;

  std::cout << "entire matches:"; 
  std::regex_token_iterator<std::string::iterator> a ( s.begin(), s.end(), e );
  while (a!=rend) std::cout << " [" << *a++ << "]";
  std::cout << std::endl;

  std::cout << "2nd submatches:";
  std::regex_token_iterator<std::string::iterator> b ( s.begin(), s.end(), e, 2 );
  while (b!=rend) std::cout << " [" << *b++ << "]";
  std::cout << std::endl;

  std::cout << "1st and 2nd submatches:";
  int submatches[] = { 1, 2 };
  std::regex_token_iterator<std::string::iterator> c ( s.begin(), s.end(), e, submatches );
  while (c!=rend) std::cout << " [" << *c++ << "]";
  std::cout << std::endl;

  std::cout << "matches as splitters:";
  std::regex_token_iterator<std::string::iterator> d ( s.begin(), s.end(), e, -1 );
  while (d!=rend) std::cout << " [" << *d++ << "]";
  std::cout << std::endl;

  return 0;
}

Output:
entire matches: [subject] [submarine] [subsequence]
2nd submatches: [ject] [marine] [sequence]
1st and 2nd submatches: [sub] [ject] [sub] [marine] [sub] [sequence]
matches as splitters: [this ] [ has a ] [ as a ]

Answer 4

You could use the suffix() function, and search again until you don't find a match:

int main()
{
    regex exp("(\\b\\S*\\b)");
    smatch res;
    string str = "first second third forth";

    while (regex_search(str, res, exp)) {
        cout << res[0] << endl;
        str = res.suffix();
    }
}   

Answer 5

My code will capture all groups in all matches:

vector<vector<string>> U::String::findEx(const string& s, const string& reg_ex, bool case_sensitive)
{
    regex rx(reg_ex, case_sensitive ? regex_constants::icase : 0);
    vector<vector<string>> captured_groups;
    vector<string> captured_subgroups;
    const std::sregex_token_iterator end_i;
    for (std::sregex_token_iterator i(s.cbegin(), s.cend(), rx);
        i != end_i;
        ++i)
    {
        captured_subgroups.clear();
        string group = *i;
        smatch res;
        if(regex_search(group, res, rx))
        {
            for(unsigned i=0; i<res.size() ; i++)
                captured_subgroups.push_back(res[i]);

            if(captured_subgroups.size() > 0)
                captured_groups.push_back(captured_subgroups);
        }

    }
    captured_groups.push_back(captured_subgroups);
    return captured_groups;
}

Answer 6

My reading of the documentation is that regex_search searches for the first match and that none of the functions in std::regex do a "scan" as you are looking for. However, the Boost library seems to be support this, as described in C++ tokenize a string using a regular expression