OS: Archlinux
grep version: 2.27-1

Bug description: grep behaves weirdly when searching hex values.

How to reproduce:
$ printf '\x53\xef' | grep -aoP '\x53\xef'
(no output, returns 1)
$ printf '\x53\xc3\xaf' | grep -aoP '\x53\xef'
Sï
$ printf '\x53\xc3\xef' | grep -aoP '\x53\xef'
(no output, returns 1)

grep (version 2.12-2) provided by Debian works just fine.