Explanation of difference between GNU sed and BSD sed

Question

I wrote the following command

echo -en 'uno\ndue\n' | sed -E 's/^.*(uno|$)/\1/'

expecting the following output

uno

This is indeed the case with my GNU Sed 4.8.

However, I've verified that BSD Sed outputs

Why is that the case?

Sherylsheryle

I'm not sure I would have the same expectations. Regexes are greedy. Because of that, the .* should always match the entire line, so that inside the parens matches the end of line. – Indistinguishable

This answer goes in-depth about the differences between various sed implementations. – Ecology

Just a guess here: it looks like the GNU ERE regex engine is willing to backtrack farther to find the longer match ("uno"), whereas the BSD regex engine is happy enough to let .* consume the whole line, and then capture ($) the empty string. – Alkalify

@TimRoberts, I'm pretty sure Mastering Regular Expressions gives examples of engines where alternation is not greedy nor lazy, but ordered. – Sherylsheryle

perl gives empty lines too. I think this depends on implementation, and as linked above, there are plenty of differences between GNU and BSD – Eolith

@TimRoberts quantifiers in BRE/ERE are not exactly greedy though, longest match wins. For example, echo 'foo123312baz' | grep -oE 'o[123]+(12baz)?' gives o123312baz whereas you'll get o123312 with greedy quantifiers like those in PCRE – Eolith

Answer 1

I'd say that BSD's sed is POSIX-compatible only. POSIX specifies support only for basic regular expressions, which have many limitations (e.g., no support for | (alternation) at all, no direct support for + and ?) and different escaping requirements.

BSD sed is default one on MacOS so very first thing on a new system is to get GNU-compatible sed: brew install gsed.

Gravelly

BSD sed is not POSIX-compatible only. BSD sed does support extended regular expressions. – Underclassman