Mention `grep -P` to get things like `\d`

The `grep -P` flag enables Perl regex syntax, which includes a lot of useful things like `\d` for digits. Running `man pcresyntax` will give them a list of all the nifty things that this includes.

This won't work on the default MacOS (BSD) version of `grep`, so folks on a Mac do need to make sure they've installed and am using the GNU versions of these tools.