Grep colors highlight all possible matches

[Pollock, Wayne]

In teaching REs, I use grep and recently noticed
the colors highlight all possible matches, not
the one match as per POSIX's rules:

  echo 'abcdefabc' | grep '[abc]'

Should highlight only the first "a" (earliest match has
precedence, than longest).  Instead, every "a", "b", and
"c" is highlighted.  It would be my guess that grep historically
didn't need to know exactly what matched, just that something
did, so the code doesn't bother to apply any rules to determine
exactly what matched.

I don't mind the current behavior as a default, but there should
be some option to force grep to only highlight the one match,
as per BRE/ERE/PCRE rules of precedence.

-- 
Wayne Pollock
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