Demo, C++ (clang) - rextester

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//clang 3.8.0

#include <iostream>
#include <regex>

unsigned int count_match( const std::string& user_string, const std::string& user_pattern, const std::string& flags = "o" ){

const bool flags_has_i = flags.find( "i" ) < flags.size();
    const bool flags_has_g = flags.find( "g" ) < flags.size();

std::regex::flag_type regex_flag                  = flags_has_i ? std::regex_constants::icase         : std::regex_constants::ECMAScript;
//    std::regex_constants::match_flag_type search_flag = flags_has_g ? std::regex_constants::match_default : std::regex_constants::format_first_only;
    std::regex rx( user_pattern, regex_flag );
    std::match_results< std::string::const_iterator > mr;

unsigned int counter = 0;
    std::string temp = user_string;
    while( std::regex_search( temp, mr, rx ) ){
        temp = mr.suffix().str();
        ++counter;
    }

if( flags_has_g ){
        return counter;
    } else {
        if( counter >= 1 ) return 1;
        else               return 0;
    }

}

int main()
{
  std::string s = "ONE TWO THREE four five six seven";
    
  std::cout << count_match( s, "[A-Z]+" ) << '\n';          // 1 => First match
  std::cout << count_match( s, "[A-Z]+", "g" ) << '\n';     // 3 => Global match
  std::cout << count_match( s, "[A-Z]+", "gi" ) << '\n';    // 3 => Global match plus insensitive     
}

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Compilation time: 2.62 sec, absolute running time: 0.14 sec, cpu time: 0 sec, memory peak: 3 Mb, absolute service time: 2,77 sec