SQL & PL/SQL :: Multipurpose Varchar2 Column Error When Converting To Date

i have a varchar2 column containing string values that can be converted to date i.e. ('31-JUL-11') and that column also contains text strings in it. i.e. ('Some string data...')

records whose column value can be converted to date are extractable via where clause (i.e. those rows are associated with some fix number / flag)

now when i try to use to_date function i get the error that

" ORA-01858 a non-numeric character was found where a numeric was expected "

in sql i have added a where clause to only pick rows with flag, but even then it gives the error.

using a subquery in the from clause eliminates the error, but when i create it in a view it again gives the same error.