Whether to escape ( and ) in regex using GNU sed

Question

I\'ve noticed several posts on this site which say that with gnu sed you should use ( and ) in regex rather than \\( and \\)

Answer 1

Originally sed, like grep and everything else, used \( to indicate grouping, whereas ( just matched a literal open-paren.

Many newer implementations of regular expressions, including egrep and perl, switched this around, so \( meant a literal open-paren, and ( was used to specify grouping.

So now with gnu sed, ( is a special character; just like egrep. But on other systems (e.g. BSD) it's still the old way, as far as I can tell. Unfortunately this is a real mess, because now it's hard to know which one to use.