`grep -P` kinda annoying. GNU has Perl-compatible regex, and BSD does not. You're reaching for `perl` or installing `ggrep` the moment you need a lookbehind.
BSD grep is the pure grep version though. Perl regex is unnecessary bloat.
Is it unnecessary bloat to match for digits X in patterns $X, say among a bunch of receipts? Eye of the beholder and all, but I'm inclined to think no.