This is a request and further endorsement of a report from 2008, ending with
https://lists.gnu.org/archive/html/bug-grep/2008-08/msg00002.html Request: Either add four words to the manpage or delete the entire sentence.
ASIS: "In GNU grep there is no difference in available functionality between basic and extended syntaxes."
TOBE: "Although the syntaxes differ, in GNU grep there is no difference in available functionality between basic and extended syntaxes."
Reasoning: The user should not suffer for not realizing that "no difference in available functionality" does not imply "no difference in syntax."
Example:
$ grep -i "([A-Z])\1\1" grep: Invalid back reference
#but no problem with:
$ grep -i -E "([A-Z])\1\1"