The backslash may be followed by a number.
In the backreference version, "\1" says to match the same string as was matched by the nth subexpression.
As a first example, we can use the backreference in a manner similar to the repeat operator.
SQL>
SQL> SELECT REGEXP_SUBSTR('Yababa dababa do','(ab)')
2 FROM dual;
RE
--
ab
SQL>
SQL> SELECT REGEXP_SUBSTR('Yababa dababa do','(ab)\1')
2 FROM dual;
REGE
----
abab
SQL>
SQL> SELECT REGEXP_SUBSTR('Yababa dababa do','(ab){2}') from dual;
REGE
----
abab
SQL>