bug#25336:

[Paul Eggert]

Zepp Lu wrote:

> $ 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)

I don't see a bug here. PCRE patterns like \xef match code points, not bytes, so 
the PCRE notation differs from the shell printf notation. If your locale uses 
UTF-8, the PCRE pattern \xef matches the Unicode character U+00EF LATIN SMALL 
LETTER I WITH DIAERESIS, which is represented by the byte pair C3 AF.

If you want \xef to match a single byte, run grep in a single-byte locale, e.g., 
set LC_ALL=C in the environment.

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

Actually, it's buggy in this area. Sometimes it can dump core.