SQL> select concat(concat(to_char(trunc(to_number('1135')/100)),':'),to_char(mod(to_number('1135'),100))) res
  2  from dual
  3  /
RES
-----
11:35

SELECT  REGEXP_REPLACE('&str','(..$)',':\1') X
  FROM  DUAL
/
Enter value for str: 1135
old   1: SELECT  REGEXP_REPLACE('&str','(..$)',':\1') X
new   1: SELECT  REGEXP_REPLACE('1135','(..$)',':\1') X

X
-----
11:35

SQL> /
Enter value for str: 345
old   1: SELECT  REGEXP_REPLACE('&str','(..$)',':\1') X
new   1: SELECT  REGEXP_REPLACE('345','(..$)',':\1') X

X
----
3:45

SQL> /
Enter value for str: 12345
old   1: SELECT  REGEXP_REPLACE('&str','(..$)',':\1') X
new   1: SELECT  REGEXP_REPLACE('12345','(..$)',':\1') X

X
------
123:45

SQL> /
Enter value for str: 12
old   1: SELECT  REGEXP_REPLACE('&str','(..$)',':\1') X
new   1: SELECT  REGEXP_REPLACE('12','(..$)',':\1') X

X
---
:12

SQL> /
Enter value for str: 1
old   1: SELECT  REGEXP_REPLACE('&str','(..$)',':\1') X
new   1: SELECT  REGEXP_REPLACE('1','(..$)',':\1') X

X
-
1

SQL> 

SQL> WITH DATA AS (SELECT 12345 a FROM dual
  2                UNION ALL
  3                SELECT 1234 a FROM dual
  4                UNION ALL
  5                SELECT 123 a FROM dual
  6                UNION ALL
  7                SELECT 12 a FROM dual
  8                UNION ALL
  9                SELECT 1 a FROM dual)
 10  SELECT a, SUBSTR(a, 1, LENGTH(a) - 2)||':'||SUBSTR(a,DECODE(LENGTH(a),1,-1,-2))
 11  FROM DATA;
 
         A SUBSTR(A,1,LENGTH(A)-2)||':'||
---------- --------------------------------------------------------------------------------
     12345 123:45
      1234 12:34
       123 1:23
        12 :12
         1 :1