`sed -r` is not present on some BSD based systems, including MacOSX
Darwin. perl is standard pretty much everywhere, so this a more
reliable way to test.
Since this is only used for tests, any performance differences
should not matter significantly.
`grep -P` gives access to the PCRE matching engine, which is nice,
however, the version of grep shipped on many BSD systems (including
Darwin) does not have this flag.
Currently, `grep -P` was being used in the `_includes()` test helper.
For cross platform compatibility in tests, do not rely upon this option.
Luckily, all existing tests seem to work fine without it already!
