Except the small detail that C's POSIX support is much wider than Rust's tier 1 platforms.
I am fairly certain that without profiling and defining a test configuration for a set of specific C++ compilers / standard C++ library I won't assert anything about std::regex performance with 4 MB of RAM.
I think the definition is meaningless. Rust spends huge amount of resources to test itself and its surrounding ecosystem on tier 1 platforms.
I'm fairly certain regex from c++ std will run like crap, if at all on something with 4MB of RAM.