Ignore illegal numbers in SQL query

Question

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]

Answer 1

This may not be the most efficient for a large result set, but you could use a function which looks something like ...
create or replace function my_to_number(strParam in Varchar2) return number is
     intPos Integer;
     notNumber Boolean;
begin
     intPos := 0;     
     notNumber := False;
     Loop
          intPos := intPos + 1;     
          notNumber := (ascii(substr(strParam,intPos,1)) < 48 or ascii(substr(strParam,intPos,1)) > 57) and
          ascii(substr(strParam,intPos,1)) != 46;
          Exit When intPos = length(strParam) or notNumber;
     End Loop;
     If notnumber then
          return null;
     Else
          return to_number(strParam);
     End If;
end;
or simply handle the exceoption in the function ...
create or replace function my_to_number(strParam in Varchar2) return number is
tempResult Number;
begin
     tempResult := to_number(strParam);
     return tempResult;
     Exception
     When Others then
          return null;
end;