SQL & PL/SQL

on 10.2 I tried to use regular expression but TO_NUMBER function does not work with REGEXP_REPLACE as below;

SQL> SELECT regexp_replace(123456,
  2                        '(.)',
  3                        '\1+') || '0' RESULT
  4    FROM dual;

RESULT
-------------
1+2+3+4+5+6+0

SQL> SELECT 1+2+3+4+5+6+0 RESULT FROM dual;

    RESULT
----------
        21

SQL> SELECT regexp_replace(123456,
  2                        '(.)',
  3                        '\1*') || '1' RESULT
  4    FROM dual;

RESULT
-------------
1*2*3*4*5*6*1

SQL> SELECT 1*2*3*4*5*6*1 RESULT FROM dual;

    RESULT
----------
       720

I recieve ORA-01722: invalid number as below;

SQL> SELECT to_number(regexp_replace(123456,
  2                        '(.)',
  3                        '\1+') || '0') RESULT
  4    FROM dual;

SELECT to_number(regexp_replace(123456,
                      '(.)',
                      '\1+') || '0') RESULT
  FROM dual

ORA-01722: invalid number

Any comments? Thank you.