static std::size_t length(const char_type* p);
Calculates the length of a null-terminated character sequence, that is, the smallest
i such that p[i]==0.
Standard library specializations of std::regex_traits execute std::char_traits<CharT>::length(p);
|p||-||pointer to the first element of the null-terminated character sequence|
 Return value
The length of the null-terminated character string.