I googled around but still cannot find the error.
Why does the following code print false, I expected true?
#include <iostream>
#include <regex>
using namespace std;
int main()
{
std::string in("15\n");
std::regex r("[1-9]+[0-9]*\\n",
std::regex_constants::extended);
std::cout << std::boolalpha;
std::cout << std::regex_match(in, r) << std::endl;
}
The option to use regex_search is not given.
There is an extra slash before the "\n" in your regex. The code prints true with just the slash removed.
#include <iostream>
#include <regex>
using namespace std;
int main()
{
std::string in("15\n");
std::regex r("[1-9]+[0-9]*\n",
std::regex_constants::extended);
std::cout << std::boolalpha;
std::cout << std::regex_match(in, r) << std::endl;
}
Edit: @rici explains why this is an issue in a comment:
Posix-standard extended regular expressions (selected with
std::regex_constants::extended) do not recognize C-escape sequences such as\n. See Posix base definitions 9.4.2: "The interpretation of an ordinary character preceded by a ( '\' ) is undefined."
true for me with gcc version 4.9.2 20141101 (Red Hat 4.9.2-1) (GCC) when I use the command g++ -std=c++11 r.cpp && ./a.out to compile and run the program.
libc++ and thus obfuscated my regex. Thanks for help!
std::regex_constants::extended) do not recognize C-escape sequences such as \n. See Posix base definitions 9.4.2: "The interpretation of an ordinary character preceded by a <backslash> ( '\\' ) is undefined."