C ++正则表达式库

Question

I have this sample code

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

int main ()
{
  std::string s ("eritueriotu3498 \"pi656\" sdfs3646df");
  std::smatch m;
  std::string reg("\\(?<=pi\\)\\(\\d+\\)\\(?=\"\\)");
  std::regex e (reg);   

  std::cout << "Target sequence: " << s << std::endl;

  std::cout << "The following matches and submatches were found:" << std::endl;

  while (std::regex_search (s,m,e)) {
     for (auto x:m) std::cout << x << " ";
     std::cout << std::endl;
     s = m.suffix().str();
  }

  return 0;
}

I need to get number between pi and " -> (piMYNUMBER") In online regex service my regex works fine (?<=pi)(\\d+)(?=") but c++ regex don't match anything.

Who knows what is wrong with my expression? Best regards

Answer 1

That is correct, C++ std::regex flavors do not support lookbehinds. You need to capture the digits between pi and " :

#include <iostream>
#include <vector>
#include <regex>

int main() {
   std::string s ("eritueriotu3498 \"pi656\" sdfs3646df");
   std::smatch m;
   std::string reg("pi(\\d+)\""); // Or, with a raw string literal:
   // std::string reg(R"(pi(\d+)\")");
   std::regex e (reg);   

   std::vector<std::string> results(std::sregex_token_iterator(s.begin(), s.end(), e, 1),
                               std::sregex_token_iterator());
   // Demo printing the results:
   std::cout << "Number of matches: " << results.size() << std::endl;
   for( auto & p : results ) std::cout << p << std::endl;
   return 0;
}

See the C++ demo . Output:

Number of matches: 1
656

Here, pi(\\d+)" pattern matches

• pi - a literal substring
• (\\d+) - captures 1+ digits into Group 1
• " - consumes a double quote.

Note the fourth argument to std::sregex_token_iterator , it is 1 because you need to collect only Group 1 values.