Matching and searching

To ask whether a given input string haystack conforms to a given regex rneedle, you can use std::regex_match(haystack, rneedle). The regex always comes last, which is reminiscent of JavaScript's syntax haystack.match(rneedle) and Perl's haystack =~ rneedle even as it's opposed to Python's re.match(rneedle, haystack). The regex_match function returns true if the regex matches the entire input string, and false otherwise:

    std::regex rx("(left|right) ([0-9]+)");
    std::string line;
    while (std::getline(std::cin, line)) {
      if (std::regex_match(line, rx)) {
        process_command(line);
      } else {
        printf("Unrecognized command '%s'.\n",
          line.c_str());
      }
    }

The regex_search function returns true if the regex matches any portion of the input string. Essentially, it just puts .* on both sides of the regex you provided and then runs the regex_match algorithm; but implementations can generally perform a regex_search faster than they...