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operator<< (std::sub_match)

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std::sub_match
template < class CharT, class Traits, class BidirIt >

std:: basic_ostream < CharT,Traits > &

operator << ( std:: basic_ostream < CharT,Traits > & os, const sub_match < BidirIt > & m ) ;
(since C++11)

Writes the representation of the matched subsequence m to the output stream os . Equivalent to os << m. str ( ) .

Parameters

os - output stream to write the representation to
m - a sub-match object to output

Return value

os

Example

Run this code 
#include <iostream>
#include <regex>
#include <string>
int main()
{
    std::string sentence{"Quick red fox jumped over a lazy hare."};
    const std::regex re{"([A-z]+) ([a-z]+) ([a-z]+)"};
    std::smatch words;
    std::regex_search(sentence, words, re);
    for (const auto& m : words)
        // m has type `const std::sub_match<std::string::const_iterator>&`
        std::cout << '[' << m << "] ";
    std::cout << '\n';
}

Output: 

[Quick red fox] [Quick] [red] [fox]