How is regex_match with char* expected to work?

The standard library defines regex_match against a simple char* string. Furthermore, regex_match returns true only when the entire string matches, yet this function doesn't take a length parameter for the char* string. How does regex_match work in this case?

The function prototype:

 bool regex_match (const charT* s, const basic_regex<charT,traits>& rgx,
      regex_constants::match_flag_type flags = regex_constants::match_default);