Without the possessive quantifier the regex engine has to go back and test every combination of .* and either 3 or 4 characters to see if it can find a matchable combination. With the possessive quantifier the regex starts where the 2nd possessive quantifier left o , the '0' character, and the regex engine tries to adjust the .* to allow \d{3,4} to match; when it can't the regex just fails, no back tracking is done to see if earlier .* adjustment could have allowed a match.

Section 70.7: Splitting a string

std::vector<std::string> split(const std::string &str, std::string regex)

{

std::regex r{ regex };

std::sregex_token_iterator start{ str.begin(), str.end(), r, -1 }, end; return std::vector<std::string>(start, end);

}

split("Some string\t with whitespace ", "\\s+"); // "Some", "string", "with", "whitespace"