grep -P fails on macOS

[hakanai]

I tried this script out at work, and it seemed to be working for some time, during which I guess it was just gathering the files to perform the operation on. Then it finally failed with:

grep: invalid option -- P
usage: ...

I gather that -P means to treat the regex as Perl-compatible, and I can't see any suitable equivalent for that in the man page, so I might just be out of luck this time.