c++ - Is boost regex slower than this combination of isupper and isdigit? -

i need check whether string consists of special set of characters (abcdefghijklmnopqrstuvwxyz0123456789<).

i either use boost regex or combination of isupper , isdigit. 1 considered better choice when comes performance? string lenghts testing around 100 characters.

bool isvalid(string& rstring) {   boost::regex regex("[0123456789abcdefghijklmnopqrstuvwxyz<]+");   boost::cmatch match;    return boost::regex_match(rstring.c_str(), match, regex); }  bool isvalid(string& rstring) {         (string::size_type = 0; < rstring.size(); ++i)             if (!isupper(rstring[i]))                 if (!isdigit(rstring[i]))                     if (rstring[i] != '<')                         return false;         return true; } 

is yyy slower yyy ?

answer: time it

in case (swapping boost::regex std::regex):

#include <vector> #include <iostream> #include <iomanip> #include <regex>  bool isvalid1(std::string const& rstring) {     static const std::regex regex("[0-9a-z<]+");      return std::regex_match(rstring, regex); }  bool isvalid2(std::string const& rstring) {     (std::string::size_type = 0; < rstring.size(); ++i)         if (!std::isupper(rstring[i]))             if (!std::isdigit(rstring[i]))                 if (rstring[i] != '<')                     return false;     return true; }  auto make_samples = []() {     std::vector<std::string> result;     result.reserve(100000);     std::generate_n(std::back_inserter(result), 100000, []                     {                         if (rand() < (rand_max / 2))                         {                             return std::string("abcdef34<63dfgs");                         }                         else                         {                             return std::string("abcdef34<63dfgs");                         }                     });     return result; };  int main() {     auto samples = make_samples();      auto time = [](const char* message, auto&& func)     {         clock_t tstart = clock();         auto result = func();         clock_t tend = clock();         std::cout << message << " yields " << result << " in " << std::fixed << std::setprecision(2) << (double(tend - tstart) / clocks_per_sec) << '\n';     };        time("regex method: ", [&]()->std::size_t          {              return std::count_if(samples.begin(), samples.end(), isvalid1);          });      time("search method: ", [&]()->std::size_t          {              return std::count_if(samples.begin(), samples.end(), isvalid2);          }); } 

sample results:

regex method:  yields 49816 in 1.29 search method:  yields 49816 in 0.04