std::regex_error on examples with gcc-4.8

[budziq]

Hi,

After clean build, all of the examples fail with std::regex_error

``` ./src/examples/example_01`
terminate called after throwing an instance of 'std::regex_error'
what(): regex_error
[1] 388 abort (core dumped) ./src/examples/example_01

After reading other bug reports I gather that gcc-4.8 is simply not supported (I know it's outdated but update is not an option for us at the moment). I suppose that the best approach would be to update the readme with minimal supported compiler version.

Best Regards,
Budziq

[raddy]

Yeah, I'm stuck on 4.8 too. Too bad.

[cjgdev]

It's a shame that the implementation of std::regex in gcc 4.8 is throwing a std::regex_error. I anticipate that the problem is most likely here: https://github.com/blockchaindev/benchpress/blob/master/src/benchpress/benchpress.hpp#L413

Unfortunately I don't have access to a gcc 4.8 compiler to hand, but if anyone is willing to debug and submit a fix I would be happy to accept a pull request.

It is possible that this bug can't be fixed, as the implementation of std::regex in gcc 4.8 was buggy, and only got fixed in gcc 4.9. The alternative is to add a new dependency on boost::regex (yuck), or selectively disable std::regex support based on the compiler version (yuck yuck).