std::match_results::prefix

From Cppreference

[edit] Parameters

(none)

[edit] Return value

Returns a sub_match representing the part of the target sequence between the beginning of the target sequence and the entire match of the regular expression.

[edit] Example

#include <iostream>
#include <regex>
#include <string>
 
int main()
{
  std::regex re("a(a)*b");
  std::string target("baaaby");
  std::smatch sm;
 
  std::regex_search(target, sm, re);
  std::cout << sm.prefix().str() << '\n';
}

Output:

b