Search notes:
ORA-12727: invalid back reference in regular expression
Ok:
select
regexp_substr('foo bar baz', 'foo bar (\w*)', '\1')
from
dual;
Also ok, although \2 references a group that does not exist in the pattern.
select
regexp_replace('foo bar baz', 'foo bar (\w*)', '\2')
from
dual;
Also ok:
select
regexp_replace('abc def ghi ghi jkl', '.* (\w+) \1.*', '\1') ghi
from
dual;
This causes ORA-12727: invalid back reference in regular expression because \2 references a group that is not defined in the pattern.
select
regexp_replace('abc def ghi ghi jkl', '.* (\w+) \2.*', '\1') ghi
from
dual;