bash - 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 ). But then I looked in the gnu sed manual and saw that they specify that ( and ) must be used. What's up?

Answer 1

This part of the gnu sed manual you linked to explains that whether you should escape parentheses depends on whether you are using basic regular expressions or extended regular expressions. This part says that the -r flag determines what mode you are in.

Edit: as stated in grok12's comment, the -E flag in bsd sed does what the -r flag does in gnu sed.