The current defacto regular expression implementation is the one used by Go, which uses the re2 syntax. It isn't POSIX-compliant, nor is it immediately compatible with C++'s std::basic_regex and friends. This shows up most obviously when trying to use flags (. matches newline, case-insensitive matching, etc.) to modify the matching behavior: Go encodes these as part of the expression string while C++ uses a separate bitmask.