Use dbms_lob.instr and dbms_lob.substr, just like regular InStr and SubstStr functions.
Look at simple example:
SQL> create table t_clob(
2 id number,
3 cl clob
4 );
Tabela zosta¦a utworzona.
SQL> insert into t_clob values ( 1, ' xxxx abcd xyz qwerty 354657 [] ' );
1 wiersz zosta¦ utworzony.
SQL> declare
2 i number;
3 begin
4 for i in 1..400 loop
5 update t_clob set cl = cl || ' xxxx abcd xyz qwerty 354657 [] ';
6 end loop;
7 update t_clob set cl = cl || ' CALCULATION=[N]NEW.PRODUCT_NO=[T9856] OLD.PRODUCT_NO=[T9852].... -- with other text ';
8 for i in 1..400 loop
9 update t_clob set cl = cl || ' xxxx abcd xyz qwerty 354657 [] ';
10 end loop;
11 end;
12 /
Procedura PL/SQL zosta¦a zako?czona pomytlnie.
SQL> commit;
Zatwierdzanie zosta¦o uko?czone.
SQL> select length( cl ) from t_clob;
LENGTH(CL)
----------
25717
SQL> select dbms_lob.instr( cl, 'NEW.PRODUCT_NO=[' ) from t_clob;
DBMS_LOB.INSTR(CL,'NEW.PRODUCT_NO=[')
-------------------------------------
12849
SQL> select dbms_lob.substr( cl, 5,dbms_lob.instr( cl, 'NEW.PRODUCT_NO=[' ) + length( 'NEW.PRODUCT_NO=[') ) new_product
2 from t_clob;
NEW_PRODUCT
--------------------------------------------------------------------------------
T9856