I haven't looked at the GNU grep source, but yes, invalid UTF-8 is one of the reasons for treating file as binary: $ printf 'a\222b' | LC_ALL=en_US.UTF-8 gr

I haven't looked at the GNU grep source, but yes, invalid UTF-8 is one of the reasons for treating file as binary: $ printf 'a\222b' | LC_ALL=en_US.UTF-8 grep a Binary file (standard input) matches This is POSIX-compliant, because "character" is defined as: > A sequence of one or more bytes representing a single graphic symbol or control code. https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_87 https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1...