How to match multiple results using std::regex - c++

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:
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
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