Score! I'm so happy to have found this page:
https://www.gnu.org/software/gnulib/manual/html_node/Regular-expression-syntaxes.html
14.8 Regular expression syntaxes
Gnulib supports many different types of regular expressions; although the underlying features are the same or identical, the syntax used varies. The descriptions given here for the different types are generated automatically.
- awk regular expression syntax
- egrep regular expression syntax
- ed regular expression syntax
- emacs regular expression syntax
- gnu-awk regular expression syntax
- grep regular expression syntax
- posix-awk regular expression syntax
- posix-basic regular expression syntax
- posix-egrep regular expression syntax
- posix-extended regular expression syntax
- posix-minimal-basic regular expression syntax
- sed regular expression syntax
