Sed – Standard Versions Where ‘-E’ is Unsupported

bsdgnuposixsed

On BSD sed, -E is the "extended regex" flag. On GNU sed, the documentation states that -r is the extended regex flag, but the -E switch works as well (though undocumented in my research).

I recall reading somewhere that -E will be specified in the next edition of POSIX specifications, but I can't find where I read that.

(Is that true? Is there an authoritative reference for that, or a user here who is an authority?)

Just how portable is the -E switch for sed?

Are there standard (i.e. POSIX compliant) versions of sed on which -E is unsupported? (Which ones?)

Why is the -E flag not documented for GNU sed?

Best Answer

GNU first added undocumented support for -E just to be compatible with BSD syntax, and the source included the comment

/* Undocumented, for compatibility with BSD sed. */

But in 2013 that was removed in this commit with the log message

Modify documentation to note sed "-E" option, now in POSIX, for EREs.

and the commit references a defect tracker for POSIX at this page that marks as accepted adding the -E flag to the sed arguments

It doesn't seem to have made it into the latest POSIX spec (sed specific part) though, but I guess it's coming.

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