SQL & PL/SQL

We have a VARCHAR2 column in a table. In a SELECT statement want to be able to do arithmetic on the values in the column provided they are valid numbers. For rows where the value is not numeric assume a zero or Null result or if that is not possible only return the rows where the value is a valid number.

Using the to_number function returns Oracle errors of
ORA-01722: invalid number

[Oracle Database is part of a package which gives us facilities to write select statements to query the package data but not write our own functions]