SQL> CREATE OR REPLACE FUNCTION PadTaxNum (x IN VARCHAR2) -- usr string
  2    RETURN VARCHAR2
  3    IS
  4  BEGIN
  5    RETURN
  6           SUBSTR(x, 1, 4)
  7    ||'-'||SUBSTR(x,5,3)
  8    ||'.'||SUBSTR(x,8,2)
  9    ||'-'||SUBSTR(x,10,2)
 10    ||'.'||SUBSTR(x,12,2)
 11    ||'-'||SUBSTR(x,14,3)
 12    ||'.'||SUBSTR(x,17);
 13  END;
 14  /

Function created.

SQL> SELECT padtaxnum('My name is Littlefoot') FROM dual;

PADTAXNUM('MYNAMEISLITTLEFOOT')
---------------------------------------------------------------------------

My n-ame. i-s .Li-ttl.efoot

SQL>

1 CREATE OR REPLACE FUNCTION PadTaxNum (u_string IN VARCHAR2) -- usr string
RETURN VARCHAR2
IS
BEGIN
RETURN SUBSTR(u_string, 1, 4)
||'-'||SUBSTR(u_string, 5,3)
||'.'||SUBSTR(u_string, 8,2)
||'-'||SUBSTR(u_string, 10,2)
||'.'||SUBSTR(u_string, 12,2)
||'-'||SUBSTR(u_string, 14,3)
||'.'||SUBSTR(u_string, 17);
END;
/

select padtaxnum(ntaxnum) from ttable where rownum <= 15;

accept u_string char prompt "Enter the value: "
select padtaxnum('ntaxnum', :u_string) from ttable where rownum <= 15;

-- create a stored function (only once)
CREATE OR REPLACE FUNCTION PadTaxNum (par IN VARCHAR2)
RETURN VARCHAR2
IS
BEGIN
RETURN SUBSTR(par, 1, 4)
||'-'||SUBSTR(par, 5,3)
||'.'||SUBSTR(par, 8,2)
||'-'||SUBSTR(par, 10,2)
||'.'||SUBSTR(par, 12,2)
||'-'||SUBSTR(par, 14,3)
||'.'||SUBSTR(par, 17);
END;
/

-- call user to enter desired value in sqlplus
accept u_string char prompt "Enter the value: "
-- or define its value directly
-- define u_string="0300074000500027000"
-- and call the stored procedure for the desired output
select padtaxnum(&u_string) from ttable where rownum <= 15;

-- if you want to pass it from another function, use a sqlplus bind variable
-- by the way you used this term, where did you get it?
variable u_string char(19)
execute :u_string := <other_app_call>
-- and call the stored procedure for the desired output
select padtaxnum(:u_string) from ttable where rownum <= 15;

accept usr_string char prompt " Enter T_Number : "

SELECT <whatever>
FROM <whatever>
WHERE <whatever>
  AND rownum <= 15
  AND t_num = padtaxnum(&usr_string);

SQL> set verify off
SQL> accept u_string char prompt "Enter the value: "
Enter the value: 0300074000500027000
SQL> select 'test '||&u_string col from dual;

COL
-----------------------
test 300074000500027000

SQL> select 'test '||&u_string col from dual union all
  2  select 'test '||&u_string col from dual;

COL
-----------------------
test 300074000500027000
test 300074000500027000

SQL> undefine u_string
SQL> select 'test '||&u_string col from dual;
Zadejte hodnotu pro u_string: 0300074000500027000

COL
-----------------------
test 300074000500027000

SQL>